I would say that applies more to the high poly. The low poly should be a shell around the high - unless the object needs to split apart for some reason, in that case, the internal areas should be modeled. Here's a simple example of an object made up of intersecting spheres. The version on the left uses 30% more triangles…
I would say no and no. The AO artifacts are due to the intersecting objects - so the way to solve that is to merge the objects or enable the Two-Sided setting. I wouldn't recommend overlapping UVs - this might cause other problems as the faces are unlikely to line up perfectly. You could isolate the UVs that are completely…