As implemented right now, Combat Focus is not supposed to be consuming uses when you attack objects or enemies that you kill with one hit. In my testing today, it seems that uses ARE in fact being consumed when you attack things that you can one-shot (tested on rats), but the abilities are not triggering.
It also might be possible that uses of Combat Focus are being consumed while fighting during the cool down period. This would explain why I went from 8 uses remaining to 5 uses remaining in between two fights.
Additionally, Combat Focus is able to be triggered more than once per engagement. I have been testing this by punching wererats. If you stay in combat for the duration of the cooldown, Combat Focus uses will engage again once the cooldown period is over.
As far as refilling uses of Combat Focus, they are not replenishing on rest or by waiting for the next IC day. The only way to refill Combat Focus uses currently is to relevel the character.