Hi. I didn't DL your script yet, i just have a little comment.
Is it really important that deactivating the mine is 'really' happening ? If you can't figure out how to be 100% sure that engineers will deactivate marked mines every time , why not simulating it ? As long as it seems realistic to the players, everything is alright to do the job.
When the engineer is walking to the mine's position, I'd just check the distance to the marked mine and when he's closer than 1.5m (or so) just play the deactivate-animation and camdestroy the mine object. You could even spawn a new object there that represents a deactivated mine (if there's such a thing).
Just my 2 cents actually. I really love your idea. We need more of these little things to make the AI look smarter.
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"Technical Note: Due to the fact that distance, getpos and setpos commands do not work on mines..."errr - roger that. then use the distance-function with the inv helipads. if the mission maker gives mines a name, can you camdestroy the mines then ?