While a unit/building is being captured, it can't be control-Ked (or if that would be too powerful, don't allow it to be ctrl-ked after already has a certain percentage of capture. E.g. if the building is at least 50% captured, ctrl-k is disabled. so it's not impossible to ctrl-k it before it gets captured but the defending player has to move quickly and the ctrl-k frees up the engineer/ACU to move on to their next target sooner.)
reclaiming a building won't prevent a volatile explosion IF the building's hp drops below -10% (so if it gets hit with a big weapon like a strat bomb or percival shot, it is likely to explode, but small weapons like mantis shots or beam weapons like novax won't do that). With beam weapons it might be easier to kill the engies first anyway.
-10% would be: for a UEF t1 pgen, that has about 760 hp, so if it drops to -76 or below, it would still explode. for a UEF t3 pgen, 9000 hp, would need to do 900 "extra" damage beyond killing it, to force an explosion. Which is very doable with a percy or a strat bomber.