No the voice call counter does not go to 0 if you use a hard reset/factory reset or the Lumia/Windows Software Recovery Tool.
(But I do not know what happens if you flash the phone to a different ROM.)
Yes.
You could inspect it on scratches on the glass or cover or border and if you can not find anything it should be new or the before owner was very careful.
In phone app dial ##634# and you will see (along with some other info) a Lifetime Counter of talk time that does not get reset with carrier account change.
I do not know of a way to tell how many hours the phone has been powered on.