When a HE charges reaches point zero while a player is disarming it, then the explosion should be delayed until the disarming is interrupted.
Overtime while disarming
So as long as you’re working on the C4 it would not explode? I don’t think that’s how bomb timers work… 
I assume you’re talking OBJ here since there is no OT in SW? It would be along the same line as the general OT concept we have right now (as long as the objective is actively being worked on, time gets extended), but in this case it would apply to a very clear timer as opposed to the somewhat arbitrary game time limit which is already fairly elastic with time added as you complete objectives. I think it would be both confusing / counter-intuitive and difficult to justify from a meta point of view: bomb timer reaches 0 and doesn’t explode, but it’s not defused yet either?!? Schrodinger’s bomb, both armed and defused at the same time? 
The moment a player stops defusing and the bomb is still armed it explodes. Defusing also doesn’t delay the timer itself. It doesn’t add extra seconds. It simply doesn’t explode as long as a player is interacting with it.
That means that in the case of overtime defusing we’re talking mere seconds. If you want some realism explanation then call it a temporary ‘safe mode’ that is enabled. Like a burglar alarm.
For the game’s excitement it adds a lot however. It means that as long as the defusing team is able to reach the bomb, they’ve got some sort of chance. It means that the bomb won’t explode in your face and you lose the entire match. It provides a bigger incentive for both teams to secure that bomb position, more tension all round.
I have no problem with any sort of overtime in SW either. As long as the overtime conditions themselves make sense and don’t stretch the game too far.
Very good point actually, that tension of watching the progress vs the timer while you defuse, wondering if you’ll pull it off in time or die trying would be gone from game-deciding defuses 