Could you please elaborate on what you think are the real desires and motives of Adam ?
We still lack a complete backstory on him but it seems you have a theory that doesn't involve the Faunus at all. I'm curious. Personally, I agree with that aspect. I feel like Adam always had some personal objectives uncorrelated to the Faunus despite fighting for the Faunus. BUT I also think Adam was genuinely fighting for the Faunus because of... well the multiple examples the show gave us on that. If somehow it turns out he was faking it this whole time, then I have to say it would be a great plot twist (and Adam should receive an Oscar) ! There is still room for it to happen for me : 1) We still don't know what type of Faunus Adam is IF he is one indeed 2) We still don't know the exact reasons why he embraced the Faunus cause (did he lose someone because of discrimination ?) and 3) Why did he say "It is time I got what I deserved" at the end of the short ?? If he was so dedicated as he always appeared to be, he would have said we wouldn't he ?
To recap, given everything the show has given us (short included) my call is that Adam has always been willing to fight for the Faunus but this call has never been definitive as I always have had doubts which have recently grown thanks to the said short and Adam killing Sienna.
I still disagree with his presumed loyalty. The examples you gave (train attack, Beacon) can't only be explained through pure loyalty. All of Adam's actions could also very well have a hidden face in terms of motives. I don't even recall Adam calling himself loyal (even if he did, he could be acting). Also, why Adam killed Sienna if he was really loyal to her ?
Why Adam wasn't shown with Hazel and co at the end of the short ? If he was willing to join them, he had all the time and the capacity to do so after he saw them from the tree.
Tracking them now makes sense. We agree Salem is the best option for Adam now but he doesn't know where she is. Only the Hazel group knows but Hazel would not accept his company because he likely doesn't trust him and they don't really need him to handle themselves, Hazel is not injured enough to be unable to fight, Mercury has not been shown to be that injured and it could be a good strategy to scatter whatever forces are after them by staying splitted (also, chances are higher for the Hazel group to be spotted so Adam would notice that and keep a way out).
Also we don't know anything about Salem's defenses. Neither does Adam. It makes sense to first acknowledge what he could face. But the question is not even here. Even if Salem's defenses are strong (which is surely the case), Adam has just to make clear he wishes to speak to Salem. More importantly, Hazel has better not be around when he does so because he would very likely ditch him and undermine Adam's credibility at a crucial moment. He has reasons to keep his distance now.