3. Screenshot doesn't really prove anything (as it stated before) - one can fake it pretty easily and it can be even automated.
The only way to prove ownership is to send small amount:
- someone send you request to approve he owns some address
- you reply with some random number (new number to every new requester) and some address to send to
- once your receive exactly this amount from address he claims to own you can be pretty sure this person is the real owner.