If, as you say, it is clear that the Conciliar Church is not the Catholic Church, then it should also be clear that the head of the false Conciliar Church can not be the Head of the true Catholic Church.
I've already laid out why this isn't necessarily and absolutely the case.
One scenario is where Montini might have been blackmailed and controlled. In that case, he could have been the legitimate Pope but his acts were not free and therefore did not reflect acts of papal authority.
Another is the distinction that these papal claimants might in fact have been MATERIAL heads of the Church but deprived of formal authority, i.e. the sedeprivationist thesis or even Fr. Chazal's sedeimpoundism.
Finally, there's the Siri thesis, in which case the See would not have been vacant through 1989
I don't know why I have to keep repeating these.