I'd probably use 'about', but I don't think it 'needs' either.
"When asked to explain his whereabouts, Peter was evasive." or "Peter was evasive about where he had been, when he was asked."
1:tending or seeking to evade; not straightforward; tricky; equivocal 2:hard to catch, grasp, etc.; elusive
Yes, that is a grammatical statement.
Yes, it is correct. I always say that! You always say that! She/he always says that! They always say that! We always say that! just to add, My teacher always says that!
It is incorrect. "I say" is the right one.
No, the correct verb to use with "you" is "were": "You were in the top third of your graduating class."