I think our paleo ancestors' stomach pH would have been much higher than most of ours, naturally protecting them from bacterial infections, parasites, etc.
No, the parasite that causes trichinosis is found not just in domesticated pigs but in many wild animals. Obviously paleo man wouldn't have been eating domesticated pork but I guess he could have gotten it from wild game, though I have no idea if it was around in the paleolithic. If so I doubt paleo man "avoided it"; before modern medicine and sanitation, infectious disease was (and still is in some places) a leading cause of death. Having said that the majority of trichinosis cases aren't fatal or even severely symptomatic. Probably they had food customs/taboos involving not feeding certain foods to children, pregnant women, and other vulnerable people after noticing their potential to cause disease. That's found in pretty much all cultures, I think.
Edit: It's true that today one of the causes is feeding raw meat to pigs (including rats, yeah) and not all species of the worm cause disease in humans, so I don't think trichinosis would have been a concern for paleo man.