Therefore, there is nothing explicit in Church teaching to apply guilt of mortal sin on attending an una cuм Mass.

This is the correct attitude.  If you are going to a valid priest, offering a valid, moral, non-indult TLM, then you've done your duty and God is pleased.  If you let the devil tempt you to over-complicate the issue, then you're making up your own rules and get into trouble.  Don't do it.

Even if you adhere to the theological opinion of sedevacantism, that doesn't necessitate that you exclusively attend non una cuм Masses, despite what some will tell you.

Just today I was listening to the following discussion with +Sanborn on indefectibility and he went on a tangent about the non una cuм issue as as essentially de fide for sedevacantism, yet, towards the end of the video he contradicts himself (around 45 minutes or so) when he states that "there is no manual on how to deal with a false pope" and then proceeds to state that what positions there are constitute "theological opinion". Therefore, there is nothing explicit in Church teaching to apply guilt of mortal sin on attending an una cuм Mass.

Actually, I was referring to the situation the other way around, of someone who is not an SV attending, or not attending, non una cuм Masses.

For quite some time, I would tell myself that the whole question is a difference of perception as to what the status of the Chair of Peter is at this very moment.  My "mental gymnastics" went something like this:

A priest wakes up in the morning.  The Pope has died during the night.  He hasn't had the news on, and he doesn't know this.  He goes to the church and says Mass.  He hasn't talked to anyone.  No one has volunteered to tell him.  It just hasn't come up.  He vests and says Mass una cuм whatever Pope it is we're talking about.  Then at the end of Mass, a sweet little "church lady" comes up and says "oh, Father, have you heard the news, the Pope died during the night".  He says "no, I hadn't heard".  Now he knows.  At the time he said the Mass una cuм, he didn't know.

That's how I was able to assist at Holy Mass at SGG in Cincinnati.  One of those two times, my father went with me, received communion from Bishop Dolan, no less.  (Incidentally, my grandmother's [his mother's] name was Gertrude.)  The second time, he had a searing headache and couldn't go, my mother and I went, we were fearful he was having a stroke or something.  He always had issues such as this, and even though the doctors say no, we are convinced he had many small strokes over the years.

But then I had to admit to myself, "you're really stretching it, aren't you, that's some casuistry worthy of the most clever Jesuit, and you're not a Jesuit".  So, with regret, I ceased assisting at non una cuм Masses.  I'm not going to go so far even as to call it "conscience" --- given how it's been misused to justify dissent from Humanae vitae and other things, the Church is my conscience, I hate that word! --- as more a case of "I'd better not".  I finally had to make a decision, and that is what it is.  I don't even begin to condemn or second-guess those who would do otherwise, in either direction, or who assist at both kinds of Masses, una cuм and non una cuм.  And I might change my mind.

As long as I've got the hood of the car up checking the oil and coolant, my father was, for all practical purposes, a sedevacantist.  I once had an SSPV pamphlet on the edge of the fireplace --- it was a sunken living room and we used it as a kind of coffee table --- and he picked it up, started reading it, and said "this is IT!  this is the truth!".  I told him, yes, it's a good pamphlet, but they go too far, don't become a sedevacantist.  Then one time a few years ago, it was either JP2, Francis, don't remember which, and he said "that man is no Pope".  It wasn't a theological discussion, just a shot from the hip.  He didn't mince words in life.  Maybe we were kind of like a low-rent, hillbilly Hutton and Mel Gibson, me without the looks, the talent, the money, the boatload of kids (I've just got the one), the alcoholism, and the philandering.  He was a sharp guy, and I miss him desperately.  Requiescat in pace.

I would go to his cryptside and pour out the shot glass of Knob Creek I got for Father's Day and he couldn't share with me, like the "brothers" do, but he'd tell me that's crazy, you're wasting a perfectly good shot of bourbon, you don't have that kind of money to waste, never heard of such a thing.  So maybe I'll have two instead of one.

So, it is praiseworthy to habitually disrespect what is supposedly a sacred rite of Holy Mother Church?

I wonder how many of those within Traddieland who claim the NOM is valid and gives grace would -- if it were the only option -- actually assist thereat?  After all, if "ignorant Catholics" can navigate such dangerous waters and still be nourished and sanctified, surely battle-hardened sages could not only obtain grace, but presumably quite a bit more grace than their ignorant fellows...

Anyone?  Bueller?