Sedevacantism is not an opinion; it is a reality.

“The fact that this opinion can be followed does not imply that the opposite is false and those who follow it are wrong.” – this is your opinion. Opinions are not equal; to act morally right one should, using his best judgement, extract a certain conclusion, if the matter isn’t morally indifferent. Some might be wrong, if they do not follow the right path of reason. “We must be sure we are right before we act; otherwise, we expose ourselves to the danger of sinning, and therefore commit sin. Hence, it is necessary to act with a certain conscience, and unlawful to act with an uncertain conscience.” (Moral Theology, op. Cit. (641)) 

It can not be said that everything that is not a dogma, in a solemn form, is just a matter of opinion and we are free to choose whatever we want and prefer (first solemn dogmatic definition was declared no earlier than the IVth century). “Many, however, confuse theological opinion with theological conclusion. A theological conclusion is a firm and certain theological doctrine which flows from principles which are derives from revelation and right reason. For example, it is certain theological conclusion that God gives all man the sufficient grace to save their souls. This fact is not directly revealed, nor is declared by the Church, but is held by all theologians as absolutely certain. It could not be termed a “theological opinion”.” (from: Fr. A. Cekada: Opinionism)

If for those who “recognise and resist” both theological “opinions” – 1) Bellarmine’s about a heretical pope and 2) that in order to be a Catholic Pope one does not have to be a Catholic are right legitimate – they could then say that they are both doubtful as well. If they wish to be consistent, they might as well act in this way:

-every second week they could say Mass “una cum” Pope Francis, then, just to not make any dogma out of this position/”opinion”, whoever wishes could be allowed to take a seat in a back pew and participate non-actively;

-every other week they could skip the formulae “una cum…” (just as if one were waiting for the true Pope to be elected) and then maybe the other part of the faithful could participate non-actively.

Additionally we can be sure that most of the people would participate actively every time, just to have their Sunday obligation cleared, no matter the explicit faith of the liturgy. You can also preach sermons that we shouldn’t make a dogma out of our “opinions”, that there is no right or wrong.