Plot hole: Zeus reassures the vast audience of deities in Omnipotent City that the God Butcherer won't reach Eternity, an omnipotent being that will grant one wish to the first person ever to reach it. This knowledge is fairly trivial amongst deities. This introduces the obvious absurdity that nobody ever tried to make use of this power to undo Thanos' damage. Also, Zeus just happens to know the bad guy's plan out of the blue.
Plot hole: The spell Magicae Maxima works, but it shouldn't have worked at all because there was one ingredient missing, the petrified spider.
Suggested correction: The spell works because Winifred believes it is a petrified spider. It also works because she believes that Billy Butcherson was her lover; which he wasn't, as stated several times in the movie: in the flashback scene, Billy mentions it several times to Gilbert, and especially when Sarah tells Winifred that Billy was her (Sarah's) lover, to which Winifred replies that Sarah was just a fling; so the spell can work with incorrect material as long as Winifred, as the main witch, believes it.