Can Twilight Step move Diana No Longer Human, she gets +2 Might "When you play a spell" but is a 3 Might unit until then?

Yes — Twilight Step can move Diana. Twilight Step resolves while Diana is still a 3 Might unit, because her "When you play a spell" +2 doesn't apply until after the spell has fully resolved.

Diana's trigger uses "play" in a trigger condition, which means the spell must have resolved (Vendetta play-definitions update; FAQ #6361). So the order is: Twilight Step resolves and moves Diana while her Might is still 3 (legal for "3 [S] or less"), and only after it finishes resolving does her trigger go on the Chain and give her +2. If an opponent reacts to Twilight Step by pumping Diana above 3 before it resolves, she would become an illegal target and the move instruction would be ignored (Rule 359.3.e.2).