If Fiora Peerless attacks a battlefield, and the enemy ambushes, does she still get double might or does it fail on the chain?

Yes, Fiora still gets double Might.

The condition — being "one-on-one" — is checked only at the exact moment she is designated as the attacker. Since it was met then, her triggered ability ("When I attack...") is already on the chain. Ambushing in a unit happens as a Reaction while the chain is closed, after the trigger is placed. That doesn't retroactively cancel or negate the trigger — it resolves and doubles her Might regardless.

This is the difference between "when" (triggered, snapshots the condition) versus "while" (passive, continuously checks). Fiora uses "when," so the bonus is locked in. (FAQ #11178, #1231)