1) Boilers are not direct combatants, this would normally make them unhuntable. But since the boiler has said "bro spin come on wtf" you could interpret it as LRP, and therefore mark them dishonorable. I'd hit him once to make him go away, if he started attacking me or didn't go away I'd just leave him, even though I could kill him there I don't want to kill a T3, especially since boilers are very important for FOB sieges. It might just be one xeno but I could potentially turn the battle in marines favor if I killed it there.
2) Well, the marine clearly does not want to fight, but it would not make sense for my character to just stop because he said that. However, I would stop trying my hardest to kill him and I'd just chase him around a little until he got to Hydro as to not break immersion or so, since just stopping the hunt because he asked me to wouldn't make sense for a Yautja. This marine would be pretty important for the marines, I'm not a huge fan of the Honor system as it makes the predators kill off the marines/xenos who are carrying that side. As well as that, I wouldn't want to ruin his round as he only wanted to shoot some funny bugs, not fight me. Preds are pretty hard to 1v1 as well anyways.
3) I am going to assume this is a few xenos being hunted by a small group of marines far away from frontlines. While I could attack the marine there, I don't want to potentially make the group have to choose between shooting at me or the xenos, which would put them at a disadvantage. I'd back off and keep an eye on them with my clan mask, if he ran away from his friends to chase after me then I'd engage him via worthy means, as he is clearly interested in fighting me and not the xenos, and this won't distrupt the xeno hunt of the small group.
I hope this answers your questions to your satisfaction.
Thank you everyone else for the support, it means a lot!