It seems like this mechanic has been pretty much abandoned by the devs, or it's WAI, so this might be an exercise in futility. But here is how I think detonators should work anyway.
They can be dodged, but not resisted.
They can be dispelled.
Once applied, they will explode after 5 heroes end their turns - ally or enemy - not counting the turn that applied the detonator. This acts as the countdown. Alternatively, they have their own speed meter (speed 150), but that might be too difficult to implement.
A critical hit on a hero with a detonator gives it a 50% chance to explode.
If other detonators exist on the same hero or their allies, there is a 50% chance for each to explode whenever a detonator explodes.
For each additional detonator that explodes in a single chain reaction, damage is increased by 20% for all detonators.
An exploding detonator has a 25% chance each to apply stagger, daze, stun, or add a DoT to the hero it was applied to.
Right now an attack that only applies a detonator is only marginally better than losing a turn due to stun.