As for 1 again, I used unity and mecanim for a few years and am using unreal’s persona for maybe 7-8 months. Mecanim is great for quick and dirty stuff but persona is way bigger and more flexible. In mecanim you can only retarget the humanoid rig and otherwise retargeting is only possible if you have the same hierarchy and bone names. Persona allows you to create a custom rig and retarget any type of character. On top of that it gives you the option to choose how you want to retarget each bone because you might want to keep some bones (like ik bones) same as they are in the original animation.
The animations are duplicated because unreal makes them into two different animations but I believe that they still reference the same source fbx so changes are propagated to all retargeted animations. I might be wrong here, I didn’t test this as much as I should.
On top of that you have montages, aim spaces, you can add curves directly to your animation graph, simple ik setups… If you are a technical animator or have technical animators on your team you/them are going to love persona. It just has a steeper learning curve because there are more things going on inside of it.