I moved Akali - Silent into a defending Diana, they responded to the move trigger with a stupefy into a gust, attempting to gust her before the +2 from her ability finalised. My question is since the move trigger hadn’t resolved, was there a combat yet? Was she targetable yet?

No — combat hadn't begun, and she was not targetable.

Combat only opens during a Cleanup when the Chain is empty (Rule 460). Akali's "When I move" trigger was still on the Chain, so the Showdown hadn't opened and she hadn't gained the Attacker designation — she was not "in combat." That means her passive ("I can't be chosen by enemy spells and abilities unless I'm in combat") applied, making her Untargetable, so neither Stupefy nor Gust could legally choose her in that window (Rules 757, 355.9.b; FAQ #6117/#10571).

Bonus: even ignoring the protection, if they stacked Stupefy then Gust on the same chain, Gust resolves first (LIFO) while she's still 4 Might, so it can't target a unit over 3 Might anyway. The move trigger resolves (+2), and only after the chain empties does the next Cleanup open the Combat Showdown — at that point she's in combat and choosable, and Diana's "When a showdown begins here" trigger fires.