a pope must be in union with all the bishops in his infallible pronouncement That is technically not so; either the Pope alone or the episcopal college in unison between themselves and with the Pope may make infallible pronouncements, but it has never happened so far that the pope would make an infallible pronouncement without the agreement of the bishops. For example, if a second Arian controversy would arise, and like in the first one, a majority of bishops would fall into heresy, the Pope could alone. if need be, correct and denounce it, infallibly.
I agree fully about the rest of your post.
He would not make such denunciation it as ex cathedra infallible. Infallibility here is not really even relevant. A pope would simply not make pronouncement of faith and morals amid a controversy. He can't do it. He would put his infallibility into controversy within the Church were he to do that. If there is no unity with his bishops then he makes no infallible statement. After the heretics are excommunicated he, in union with the faithful bishops conceivably could. An infallible pronouncement also has to be of something that the faithful have always believed.