If the infection was really bad you might consider removing the topmost layer of soil in the bed and replacing it with fresh mulch or topsoil. For next year see if you can improve the airflow to make the spot less muggy and humid, and use something like a bicarbonate solution to make the leaf surfaces a less friendly place for the spores (a quick google should give you a recipe). Avoid irrigation that sprays water around - a dripper type hose is better.

I wouldn't worry about feeding them now - most people over-feed anyway. And yes, they will return next year.

DON'T feed them now. It's too late in the season, and wet, overly fertile, cold soil can make the rot worse.

If they turned brown and died during, before, or immediately after blooming, rather than around now as is normal, those bulbs might be too rotted to come back next year. If that indeed turns out to be the case, I sure wouldn't plant any more lilies in that spot any time soon, but that's just my own experience, I don't know if the rot is contagious through the soil. Or you could replace the soil as kayman advised. Good luck!

Jeanne