Lots of content we didn't get to see and I think I'll go backwards, from the end....
Alliances and the endgame:
Alliance can only be forged if we had the "Nemesis" codeword, which can be gained in various way, all of which involving Gaia giving a warning that releasing the Heart's power will destroy the world (in our case, she was warning the Baron, but we overheard it through our recording device). If we don't have that knowledge, the PC won't look for allies.
Anyway, here's a breakdown of each potential ally and what they bring:
Vajra Singh:
Gaining an alliance with this guy is harder than the other two. As you saw, there was no way to directly approach him. In fact, it was impossible for us to gain his alliance in the particular path we took.
I mentioned that there was a possibility that Janus Gaunt could be dead by the last day of the game. Basically, if we chose to confront Gaunt about his "weakness" (that he wasn't as ruthless as the other contestants), he will order his xoms to seize us and try to murder us. His theory is that since he sees you as a friend, by murdering a friend in cold blood he will expunge this "weakness" in himself. He admits that he's probably mad (and that one has to be mad to pursue the Heart's powers).
.....Or at least, that's what he'd do if we didn't have Gilgamesh with us. Apparently, he's not mad enough to mess with us if we have a bodyguard, so he simply disappears that next day (probably realizing that he's not going to win).
If we
didn't have our robot bodyguard though.....Gaunt still won't win, because Vajra Singh arrives and saves our ass by killing Gaunt. Not out altruism, but because he's pissed off that Gaunt broke the truce (he's a stickler for rules like that). That is our only opportunity to gain an alliance with him. So we never had a chance, with Gilgamesh on our side. Not that it's a bad trade-off, since Gilgamesh is certainly more useful than any human ally (without him we would have died to that phantom thing).
Anyway, as Starmaker mentioned, he's the most powerful opponent, but he's surprisingly not that brilliant as an ally. But at least he's a trustworthy one, since unlike Boche he won't try to cheap-shot us (but we can choose to cheap-shot him!).
Golgoth will be our biggest opponent in this scenario (Boche is a non-issue if we ally with Singh.....he dies immediately from the mantramukta cannon when Golgoth uses him as a meatshield to eat the first shot). He lacks the sheer firepower that Singh has, but he's more cunning and resourceful. If we choose to stay out of the fight, he will hide himself with a gas grenade, set up his gun to remote-control fire from a different spot as a decoy, and shoot Singh in the eye with his trusty crossbow.
We can choose to help our ally here, but there's a good chance that we'll become the target of the crossbow bolt instead! There're a few ways this can play out depending our skill and options, but basically Golgoth is the most tricky opponent to defeat. Alternatively, we can turn on Singh and kill him easily (because he doesn't expect us to turn on him). We then have the option to use his cannon to blast Golgoth and become the last survivor, or choose to stop the fight, in which case the story proceeds as if we'd allied with Golgoth.
While not an awesome ally, Vajra Singh is honourable to a fault. Once the fight is down to the two of you, he makes it clear that he has no intention of destroying the heart, and offers you a fair fight by putting down his OP cannon. If you don't even have a gun, he drops his as well and fights you hand-to-hand. Not that it'd be an easy fight even then - you lose 9 Life points in the ensuing fight if you don't have CLOSE COMBAT (and 5 if you do), so there's a good chance that you can die from that!
If you out-shoot him in a fair gun-duel, the text mentions he could have hit you with a dying shot but chooses not to do it - he takes his loss like a man.
Chaim Golgoth:
Not as honest as Vajra Singh, but not quite as treacherous as Kyle Boche, Golgoth is the ultimate pragmatist. When you reveal what you learnt from Gaia to him, he agrees that destroying the Heart is a top priority because it's a threat to his employer, the US government (he doesn't care about the Universe.....the Universe doesn't sign his pay cheques!)
There's one thing he certainly does better than us: finding the best possible use for Kyle Boche. We joked about using him as a meatshield at the beginning of the story but never got around to actually do it. Golgoth does it in more than one scenario. If we teamed with Singh, he immediately grabs Boche and throws him straight into Singh's first cannon-shot. In the scenario where we're allied with Golgoth himself, the first thing he does is to grab Boche, hold him in front of himself as a human shield, put a gun to his head, and order him to shoot Singh. The shot isn't enough to kill Sing (who's heavily armoured), but once Boche has weakened Singh with the shot, Golgoth coolly shoots Boche in the head before proceeding to continue his attack on Singh. If we join the fight, Singh goes down quickly. If we chose to stay out of the fight, the 2 of them eventually kill each other and you are left as the last survivor (again , Singh gives up the chance to shoot you before he dies).
When it's down to you and Golgoth, there's minor continuity error: the book automatically assumes that you are willing to destroy the Heart with him (even if you didn't have "Nemesis"). As mentioned, Golgoth is the only one who will help you destroy the Heart, and if you have the means to do so (you just need a charged gun.....Golgoth has his own), you will destroy the Heart, which bursts and kill you both in the process.
Unfortunately, if you don't have a charged gun (or a battery unit to charge Boche's empty gun), Golgoth, who realizes that you can't destroy the Heart, quickly re-evaluates his priorities, and comes to the conclusion that the next-best option is for him to take the power himself (he doesn't want to just leave it lying around for someone else). In this case, you don't stand a chance: he doesn't waste time showing off like Boche, and just "regretfully" shoots you before unleashing the power.
Like I said, a most pragmatic guy.
Kyle Boche:
Despite me continually ragging on about how useless he is as a companion, he's actually the best ally for the endgame if you don't want to destroy the Heart, simply because he's the easiest to beat once it's down to the two fo you, and he manages to manipulate Singh and Golgoth into killing each other right away without you having to do anything. I still stand by what I said about him as companion though: other than providing a means for you to steal 60 scads (which is the biggest pile of cash loot we can find in this book), every other useful thing he does happens automatically as part of the plot (getting rid of Siraisis, and the other 2 rivals) whether he's your companion or not. So if you don't have ROGUERY there's absolutely no point in accepting his companionship. If you DO have the skill, you should kick him to the curb at the first opportunity after you've looted that 60 scads.
And as you can see, he never intended to help you destroy the Heart despite his promises (at least Vajra Singh doesn't pretend to want to destroy it; the text makes it clear that he isn't interested at all in Gia's warning. He accepts the alliance only because it gives you both better chances to make it to the end).
No allies:
If you didn't make any alliances, you are given the option to strike first with a list of available weapons. If you choose not to make the first move, the scene plays out
exactly as if you allied with Boche (with him manipulating Singh and Golgoth into killing each other).
If you make the first move, you have to deice on the weapon (and if you don't have any, you can't attack first). The stasis bomb is the most powerful, taking out all three of them and freezing them, leaving you open to the Heart. But that means you are denied the option of using the bomb to make yourself the Heart's guardian after that.
There's also a stun grenade listed as a trap option: it knocks out Golgoth and Boche, but Singh is protected by his armour, and promptly blasts you out with his cannon since you're the only threat left.
If you choose to use a barysal gun, you need to pick your target. Golgoth is a death option: you need him to help take down Singh, who's got superior firepower and arms. If you shoot Golgoth down first, Singh blasts you immediately. Shooting Singh or Boche are both valid options, and leads to one of the above situations described - Singh and Golgoth killing each other.
Anyway, here's one of the pictures I left out - finding the room with the Heart:
The other rivals:
Janus Gaunt has more or less been summed up above. During your journey, you can actually learn about him in various ways, but I don't think you actually meet him in person until Du-En.
Thadra Bey can be encountered if you chose the most expensive hotel in Venis. She apparently from some space-colony named al-Lat in the Earth-Moon orbit and was looking for a contact for some information on genetic engineering and retro-viruses.. If you have STREETWISE (or and appropriate item that temporarily give you that skill....more on that later), you can provide her some information that she was looking for. She will give you some antidote pills as a way of thanks (and those can save you from radiation sickness). If you were looking for information about the Heart in Venis, STREETWISE will also tell you that both Janus Gaunt and Tharda Bey are looking for similar information.
The Gargan Sisters can only be encountered on the jungle route:
They were apparently terrorizing the local inn, and you have the option to fight them, outwit them by drugging them (using CUNNING) or just keep a low profile and snoop. We can use either ROGUERY or ESP to find out more about them. If we overhear their conversation, we learn about Gaia's warning (another way to pick up "Nemesis") about the Heart's power destroying the world, and also that Vajra SIngh and Janus Gaunt are looking for the Heart.
If we fight and win their respect with CLOSE COMBAT, they will offer to travel with you.....except it doesn't lasts, and at the first opportunity they steal your supplies and bail while you're asleep. There's no way to part with them on friendly terms.....
......which is a pity because they're the only other people besides Golgoth who actually want to destroy the Heart instead of taking power for themselves. There's never an opportunity to forge alliance with them, because they always die before the end.
If we chose to explore Du-En on the first day, we'll be going with Golgoth and the Gargans. The Gargans will object you bringing Gilgamesh ("no-servant" clause in the truce). You can agree to leave Gilgamesh behind or insist on bringing him along. If instead you decide to go find Vajra Singh to be the judge, he tells you to go sort out your own arguments, and Golgoth takes the opportunity to disable Gilgamesh with a bullet to the head while you're away.
If you manage to bring Gilgamesh along, he helps you get past some obstacles with ease, but you also risk losing him (which leaves you defenseless against the phantom-creature later), so it's probably better to leave him behind. Anyway, Golgoth will eventually goad the Gargans into attacking by revealing that he's the one who killed the rest of the brood of their sisters (there were 12 others) many years ago. The Gargans will take him down, and he will play dead. When they turn their attention to you, Golgoth will surprise them with poison needle a kill them both. Before that you have the opportunity to interfere and try to save Golgoth.....but he doesn't need you, and you shouldn't try, because you might get hurt. And if Gilgamesh is there, he may die too (he had to take off his outer armour to traverse some narrow tunnels, making him more vulnerable to attacks). After the sisters are dead, you can loot some supplies from them, including a stun grenade.
Baron Siraisis deserves a post by himself, so I'll continue tomorrow (and also the other questions I haven't yet answered.