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…
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…