So your saying that to go to the New Mass is 1.) ok ; 2.) SSPX, ok ; 3.) CMRI, ok ; 4.) Stay Home, ok ; why? Because "no one" can declare Pope Francis a "heretic"? And we are all in communion with each other as Bishop Pivarunas says?
Does some have to declare to a person that is guilty of mortal sin that it is a mortal sin, for it to be a mortal sin?
No one is saying that, or even inferring a suggestion towards the direction your implying. Quit putting words into other people's mouth.
Neither you or anyone can "declare him" as a heretic, he is already been judged by God. To imply that you would juridically declare him to be a heretic, it would imply that you are the superior of a Pope as only superiors are capable of making a legal declaration on matter's such as this. Which is why St. Bellarmine went over this matter pretty well, since he already stands condemned by his own deeds/words. The only difference is that the SSPX and other people who are currently erring in that matter will have to see, what we see by the eyes of faith (spiritual reality). Any other way of looking at this problem like how you are suggesting is condemned anathematized heresy of Conciliarism (the evil you were trying to avoid in the first place, the irony huh!?).
You don't know the state of someone else's soul, unless you are able to read heart's. Now if someone has publicly committed a crime that in the external forum is mortally sinful, sure you can be able to rebuke them about that.
Going to the New mass is false worship, it is simply a no no with the SSPX. They never recommend that, attending mass with the SSPX is doable if they are the only one in the area. CMRI would be fine to attend also, and staying at home would depend on the situation. Once again this is something that you need to discern within your particular circuмstances. Suppose that you are attending a mass with a SSPX that holds positions that are Novus Ordo, i.e. does not hold the SSPX positions. Then it would be licit to not attend where they might be more FSSP then SSPX. Just use your common sense, that's all. Just take every particular case on an individual level, don't assume anything. Simply talk to your local priest, and discern. Wish you the best, God Bless.