i dont think mac is actually that stupid or heartless enough to throw pippy under the bus to an group he is not invested in whatsoever, much less one as unsavory to be in as the gunners... but for the au's sake, i think another factor that is altered in that universe (besides the gunners rlly gunnin' for pippy's head) is how more profoundly his upbringing has left him hyper-independent and afraid to trust others. rather than acting the way he does out of this need for keeping his own and himself above the water, his behaviors are maybe moreso out of cowardice and abandonment of principle/morals.
i think a possibility could be that, he abandons these aforementioned things because he genuinely feels like those are privileges that people like him (no support system, dependents relying on him, not wealthy, no connections, no resources, etc) are not afforded, and survival at any cost is the way the world works; so if he wants to do right by the sad few he has, to keep things from falling apart... he'd have to make sacrifices. but, the unfortunate thing is that instead of the typical, honorable/glorified idea of sacrifice, he'd chosen to sacrifice his heart & do the things he does in this au, in a useless, self-sabotaging attempt to get what he wants badly.
a theme i wanted to haunt 701au mac with, is guilt. guilt for being a liar, for playing pippy/lucy/duncan for fools with his charm, and sweet, wishful words, but his actions (and ultimately the results of them) being quite contrary.
while he played along with their odd intention to help him, he maintained a constant suspension of belief that any of their positive intentions or feelings were real/justified, no matter how deeply their relationship actually impacted him. a part of him yearned for their relationship to be real (which, it WAS and could be), but there was almost a sense of learned helplessness which trapped him from that future/reality. so he shoots himself in the foot, metaphorically.
being repeatedly taught by the world that you *just can't* take risks like that, and that those won't ever amount to anything good, leads him to not bother trying to take risks for a change (to enact the future he wanted deep deep down), and it is ultimately his ruin. yay!