Your headcanons are fascinating and I'm not talking about it as in a kink, is just that they feel really... real, like is both interesting and horrible how human nature can be so tainted and ruthless, and I think it follows a part of the lore in UT anyway, so is very in universe to me, honestly I'm really amazed that Sans has been able to hold up, since monster's bodies are so dramatically fragile towards ill intentions, is really sad, but quite thorough. Well done.
ahhh, thank you so much :> the way i figured it, sans survives all the horrible beat downs because in the end the cops don’t want to (or at least know they can’t) kill him because that would just lead to problems, in addition to which sans has a reason to want to stay alive - he has a brother to look after. so as intent truly is magic with monsters, as long as the cops don’t actually intend to kill him and he doesn’t want to die, he’ll live through the abuse.
now if his attacker/s actually wanted him dead and he had no will to live, that’d be a different story altogether.