1. How do the epic level spells you can get from feats work? Casting limit, or is it infinite like an invocation?
2. Is this build: Warlock 9 Heartwarden 10 Hellfire Warlock 3 Child of Night 8 workable?

Once per day per spell. I'm not familiar with these custom prestige classes, but generally the Warlock is a weak class for epic level play. It should work, but don't expect anything special out of epic Warlock.