One other benefit you'll see from merging objects in the low poly mesh is reducing aliased geometric edges. Anywhere that you have objects intersecting like this you'll see this kind of aliasing. But if you merge the objects, the seam between them will be anti-aliased in the bake and look cleaner.
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…
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…
Cool - yeah so the issue here is if there are intersecting meshes, some of the rays can end up inside the mesh at the borders. If double-sided isn't on, the rays will shoot through the surface. Creating water-tight low poly meshes without intersections may help to avoid it too. This will also tend to reduce the triangle…