OK, if i play "Devil we know"
Target defender you control gets +2 atk this attack. If that character has the doom affiliation, whenever an attacker or defender becomes stunned this attack, ko that attacker or defender.

Could my opponent recover his char with an effect if it was going to "die" in the combat?
If I sacrified my character to any effect, after he/she/it is stunned, does he/she/it still "defend" or attacker is readied and undamaged?

As long as the opponents character becomes stunned, it will be ko'd regardless of whether or not it's recovered before DWK resolves. Having said that, your opponent can respond and ko their character (or return to hand/remove from game etc.) to another effect after the stun but before the DWK ko.

You can do the same with your attacker/defender.

DWK triggers on the stun (conclusion of attack), thus you may use another effect on your character without worrying that your opponents attacker will ready to attack again.