Does Fiora, Peerless check 1v1 only at trigger time like Kha'Zix?

It works like Kha'Zix. The 1v1 clause is part of the trigger condition checked when Fiora becomes an attacker or defender. If she is 1v1 then, she triggers and resolves at 6 Might even if a second defender arrives later. Master Yi's passive applies immediately when she becomes a defender, so Yi's +2 is active before her trigger doubles her Might.