The other obvious weak point is that you can’t surveil the guards 24/7 to make sure they don’t swap the live explosives collar with a placebo collar.

The obvious countermeasure is adding tamper detection to make it automatically trigger if they try.

Right, that’s what they do in sci-fi books and movies. But then those collars magically never need batteries replaced, service, or replacement. They also don’t have any issues with false positive detonations.

If the collar can be deactivated to be removed for servicing/off duty/retirement/replacement/etc then it can be hacked and replaced with a placebo.