Sad to say that poche is CI's shining example - and should be used as such - as to what happens to those who go to the NO. Use him as our example as to what happens to those who attend the NO, hence why we must never attend it for any reason whatsoever lest we too become infected with the same liberalism and lose our faith without even knowing it.
Can't agree more with my brother Stubborn.
I have a friend who converted to the "N.O. Church" many years ago and only recently -- within the last 18 months or less -- discovered Tradition. I have begged her to stop reverting to the N.O.M. whenever locating a TLM doesn't suit her schedule. (I'm talking more about weekday Masses, even, when there's no requirement to attend Mass at all.) She lives in a state of confusion -- although in some ways I think it's far worse to go in and out like that than to stick with either one or the other. The reason for the confusion is obvious to us, not to her, so I explained that once she starts exclusively attending the TLM, she will lose any sense of equivalency between the two, and her theological sensibilities will re-align, as will her spirituality.
Face it: these are two different religions. I was going to add "practically," but when you consider that "formally" really refers to the inherent structure and defining intellectual concepts of a body of beliefs, then it is more than just practically that New Church differs from Tradition. Practically and formally, there are two different sets of beliefs and practices. Practically and formally, New Church is not the Church of Tradition, not the Church Continuous, not the Church One, not the Church recognizably and objectively Catholic. It's just that those differences have not been announced
formally. They exist
formally, but without formal acknowledgement
Regarding the question in the OP, I don't think it's a mortal sin simply to attend. However, it could be a venial sin against the First and Third Commandments if you know in your heart that attending at least does not benefit you spiritually in the way that an available TLM does, but the TLM time or location is inconvenient. In any case, it is a foolish idea. I used to attend if I had no other choice. I won't do that now because it harms my soul and scandalizes me. It is much more fruitful for me to follow interiorly a streamed TLM, to read spiritual material, listen to traditional sermons, pray (of course!), and engage in the spiritual and corporal works of mercy on Sunday if I truly have no other choice. I recognize that all of those practices do not equal attendance at the Traditional Mass, and so I'm conscious of grieving that loss when I believe I have no other choice, but it is still, for me, a better choice than going to a N.O.M. and not being able to avoid seeing people engage in people-watching, attention-getting, Protestant behavior, and other forms of irreverence. It's sacrilegious to me.