QCan a follower have negative attack?

AYes. A follower's attack can be reduced to a negative number by effects. A follower with negative attack doesn't deal attack or combat damage.

QOn my 4th turn, I play Marwynn, Omen of Repose's "Skip your next turn: Evolve this follower" ability, increasing my max play points by 1 due to its On Evolve ability. On my next turn, which happens after 2 of my opponent's turns, does this follower cost 6 less?

ANo. It costs 5 less. Although you would have 6 max play points, skipped turns do not count towards turns passed.

QWhat does "turns passed" mean?

AA player's turns passed is the number of times they have begun their start phase as the active player for the first time in a turn.