While not necessary, the enclitic "-ne" is often used at the beginning of the first word of a "yes/no" question. So, the preferred answer should be "venitisne ab urbe" or "venisne ab urbe" (the verb also often came first in these questions).
Agree, it is surely the normal way of asking a question using just the verb
There is no indication of whether this is meant to be a singular or plural "you".
Ab urbe venitis IS correct.