Is it legal for someone or an attorney to hold another persons mail as proof of residence in a small claim dispute?

No, it is illegal (unless it is done by the parent of a minor child).

No, keeping someone else's mail is an offense with the US Postal Service.

There should be a relevant and authentic certificate to prove. In the majority good awarding bodies use a special water marked paper to print award certification on.

yes

yes if they have the password