I've updated 1, 2 and 4 but I'm not fully following on 3.
3) Really harsh angles between polygons (such as right angles) should probably have their smoothing groups and UVs split in that area. Any gradients you see in your high or low will bake into the normal map which reduces the quality of the map.
In my high poly, I have those brackets or frames that surround each face. Where the corners are for each of the frame are you saying that they need to be in a different smoothing group?
I'll illustrate what I mean

So the inner edge of the frame between the red highlight and the other that isn't highlighted both should have different smoothing groups?
Same question but on the outer edge on the next image:

Also I've exported the high poly as an fbx with Smoothing Groups, Tangents & Binormals, TurboSmooth, and Triangulate enabled to Substance Painter but when trying to bake onto my low poly mesh it gave me these error messages:
