If I managed to reveal 1 Whiteflame Protector and 1 Deadbloom Predator with 2 Dazzling Auroras, and I choose to play the Deadbloom Predator to an occupied battlefield and the Whiteflame Protector to the base, can I give the 8 might bonus to the Deadbloom Predator before starting the showdown?

Yes. The Whiteflame Protector's "+8 might this turn" trigger fully resolves and can buff the Deadbloom Predator before the showdown starts.

Sequence: the first Aurora plays Deadbloom to the enemy battlefield, staging (but not starting) the showdown. The second Aurora then plays Whiteflame Protector — to your base, since you don't control the contested battlefield. Whiteflame's "When you play me" trigger goes on the chain and resolves while the showdown is still only staged, so Deadbloom is a legal target and gets the +8. Only after the chain fully empties does the showdown open (Rules 323.12–323.13).

Two notes: the buff is +8 Might this turn, so it applies for the entire showdown/combat. And the opponent's battlefield is not under your control yet, so Whiteflame itself must be played to your base (or a battlefield you control) — you cannot play it onto the battlefield Deadbloom is contesting.