Can Fiora, Worthy, be triggered with no available yellow power, triggering Fiora Legend at the same time, resolving Fiora legend to channel a yellow rune and then pay it for the Fiora, Worthy cost?

No — the rune arrives too late. Fiora, Worthy's "you may pay [C] to ready it" is a cost within instructions sitting right after the "you may" at the front of the effect, so it's the base cost of the triggered ability and must be paid to finalize it onto the chain (Rules 383.3.b.1, 204.3.a).

With FEPR, every pending item is finalized in order before anything resolves — so you'd have to pay the power while both triggers are still sitting pending, and Fiora, Grand Duelist's ability doesn't channel anything until it resolves. If you can't pay at finalization, Fiora, Worthy's ability is removed from the chain and counts as never having triggered (Rule 383.3.a.2 / 404.2), so you can't come back and pay with the rune you just got.

Two related notes: the channeled rune enters exhausted (Rule 430.2), so it can't be used for its exhaust-to-Add ability that turn — you'd have to Recycle it for its Order power, which is legal at cost-payment time (Rule 204.4.b.1) but only matters if the rune was already on the board when you needed to pay. If you'd had any Order rune on board beforehand, recycling it would have paid the cost fine.