Unless you have an older carbureted vehicle, engine braking should use no fuel:

https://en.wikipedia.org/wiki/Engine_braking#Limitations

Just do a decent job matching revs on the downshifts so you don't trade brake dust for clutch dust :)

Not all hill holds are created equal. On my car, the brakes release as soon as you start moving forward after hill hold, or after 1.8 seconds. On my friend's subaru, the brakes hold for 2 seconds period. That means if you can do a hill start in less than 2 seconds, you drag the brakes every time.

15 civic SI. Hill hold is 1.2 seconds I think and no early disengagement. But it's rear brakes only. You can't disable it that I know of.