I’ve actually tested it now, doing Onyxia (Heroic) with 6 dragons again and this time taking Deathwing with the Demon Soul with me. So, it’s my recollection or description of that interaction that was a bit inaccurate: if the summoned minion is supposed to attack one of your characters (‘Into the Whelp Pit!’ or ‘Drakonid Rush’ in particular do that), you take control of it immediately, but then it still attacks its initial target. For this boss fight in particular, this leads to a hilarious chain reaction: your minions’ attacks (if I understand correctly, yes, it’s those whelps that you’ve taken control of) cause Onyxian Warders to spawn more whelps… there are so many that some of them will not fit on your side of the board and die immediately (for Onyxia, apparently, fuelling her subsequent ‘Volcanic Fire’), so this method is still not recommended, if you ask me.