Refutation #1 from Archbishop Lefebvre, quoted by Sean himself many times against whom he calls “Pfeifferites”:
Archbishop Lefebvre: Those who feel themselves obliged in conscience to assist at the New Mass on Sunday can fulfil their Sunday obligation. But one cannot accuse a person of a grave fault because he prefers not to assist at Mass on Sunday rather than assist at the New Mass. (Letter of May 9 1980) [II]
Major Premise: Archbishop Lefebvre could not have said those who feel obliged can fulfil one’s Sunday obligation at an NOM if it was always a mortal sin.
Minor Premise: Archbishop Lefebvre did say exactly that.
Conclusion: Ergo, it is not always a mortal sin.
Similar answer to #2. I started at #2 because the principle comes out more clearly. Notice +Lefebvre's choice of phrase. He's referring to those "who feel themselves obliged in conscience to assist at the New Mass."
He's saying that it is OBJECTIVELY not true that it's necessary to attend it to fulfill one's obligation; otherwise he couldn't possibly excuse someone from grave sin for NOT attending it. He's referring to those who "IN CONSCIENCE FEEL THEMSELVES OBLIGED". In other words, he's disagreeing with their conscience, saying that, despite how some of these people may feel, their conscience is mistaken and Catholics are not in fact objectively obliged to do so.
So, once again, for those to whom he refers, it would NOT be sinful based on the state of their conscience, but that does not speak to the objective sinfulness (or lack thereof) of the NOM.