The big question about the branch is whether or not it was green before the big bend in June. If it was green earlier in spring, there’s a good chance that the bend stressed the tree and there’s little to do to help with this.
If the branch was weak from the start, it could be that the bend exacerbated the situation or that something else is going on. Have you checked for spider mites?
Some light thinning could help ensure that the lower branch is getting enough light, but I would only want to make cuts that I’d planned to make in fall anyway. Based on the last photo, it looks like the tree can be positioned in a way that provides plenty of light to the lower branch.
As for whether or not thinning could redirect resources to the lower branch, I’d want to know more about why it’s weak in the first to know if that would make a difference.
I’d be surprised if overwatering were the problem as that typically affects the whole tree, but if there is a pocket of old soil, that alone could account for the branch weakness and be exacerbated by overwatering.