1) Does your skin and hair have to be dry before doing wudhu? What do you do if you start wudhu but mess up something so you have to restart, do you dry your face and arms, head and feet etc?

2) Also, if I touch something najis can I just wash my hands or do I have to redo wudhu

1- Only the head and feet have to be dry before wudhu. The face and arms can be wet. Not if you have to start over wudhu and your head/fees are wet, you have to dry them then complete the wudhu.

2- If you touch something najes you don’t have to redo your wudhu. Just wash your hands.