Don't panic....all is not lost! While it is best to allow the foliage to dry and die back naturally, as long as it has remained in place for a good 6 weeks after flowering, that should have allowed sufficient time for those leaves to photosynthesize and recharge the bulbs for next season. You should be okay.

I've never found it necessary to fertilize narcissus so can't help you there.

I just want to congratulate you in No Mow May!

They'll be fine:)

Yes, don’t panic! My husband has done the same on several occasions, once even while the stalks had no yellowing, and they came back in fine bloom the next year. They probably won’t multiply for next year, although they might, but they most likely will bloom.

In my zone collapse is eminent once the first yellow appears, so that would have been perfectly fine.