Hi, I do not own the device but I've been in Sony and Android ecosystem for quiet long. So I can probably answer most of your questions.
1. Its not harmful at all! In fact since Sony devices don't get hard bricked you can't go wrong with it!
2. You can, once rooted! Ofc, you'll need mod of some kind.
4. Speed issues? Probably not! But you might lose several functionalities, major one being the Camera quality. As Sony camera works as great as it is on stock only.
5. This forum is your tweak store.