If I am defending with a Fiora Peerless and my enemy attacks with 2 units but Starcross one of them away, does my Fiora Peerless trigger at any point?

No, Fiora's ability does not trigger.

Fiora's "When I attack or defend one on one" checks the one-on-one condition only at the exact moment she gains the Defender designation — when combat first begins. At that point, there were 2 enemy attackers, so the condition wasn't met, and the trigger never goes on the chain. (Rule 383.4.f.2.b: if the trigger condition contains other requirements and they aren't fulfilled when the unit gains the Defender designation, it won't trigger.)

Removing one attacker later with Star-Crossed doesn't help — Attack/Defend triggers are checked once per combat (Rule 383.4.f.2.a). Fiora already missed her window.