There is a middle case, which is more difficult to ascertain:

A law which runs contrary to the common good is no law at all.

So, since the case of many of the disciplinary changes since Vatican II do seem to run contrary to the common good (eg., 1 hour “fast;” etc.), are they in fact “law” at all?
And if they are not true laws, then the “old” laws would remain in place.

This would be an argument in favor of the old laws remaining obligatory (at least for those conscious of the evil in the new norms).

The SSPX has even acknowledged in one of its articles of a few years ago that the new norms governing fast and abstinence are not sufficient for obtaining salvation (ie., run contrary to the common good).

Confusingly, however, they do not say these new laws are not laws at all, but rather encourage the faithful to observe the traditional norms (while not saying they are obligatory).

In the end, I’m not sure what the answer is, but it sure seems safer to disregard the new norms and stick with the old, regardless of whether they remain obligatory or not.

Therefore, I accidentally eat something 2.5 hours before Mass, I do not receive communion.

I would not take communion in the situation you describe, I wouldn't feel right about it, but I wouldn't bind someone else's conscience if they decided to do so because I wouldn't think I had the authority to do that, and I wouldn't think they were committing a sin necessarily.  But, like you, I think it safer to follow the older rules.

The SSPX's position on fasting makes perfect sense to me too.  Yes, 2 days fasting and a few days abstinence a year obviously isn't enough penance, but only the Church can bind people to *specific* acts of penance.  The SSPX cannot essentially function as Pope and demand *specific* actions of penance from people that the Church isn't.  That the law is more lax than it used to be isn't sufficient reason either, since the 1958 laws were laxer than earlier, medieval rules and whatnot.

My husband booked a flight and planned a trip on Ascension Thursday but hadn't looked at the liturgical calendar to see that he would be missing a holy day of obligation that day. After realizing his mistake, he asked our SSPX pastor what to do and he responded that "they moved the obligation to Sunday", so no need to worry.

This is the problem with the one foot in the conciliar church and one foot in Tradition approach.
If the Society would just announce that they are completely in the conciliar fold, it would clarify much, but I doubt they would lose many faithful over it.  

This is why the SSPV's stance makes way more sense.

I think the SSPV's position makes sense based on a different premise.  If SSPX has any doubt that the conciliar claimants are real popes, they settle it in favor of the claimant.  If SSPV has any doubt, they settle it *against* the claimant.

In other words, SSPV might give lip service to the idea that you *can* believe Francis is a Pope if you must, but for all practical intents and purposes, they're Sedevacantists (even if they're charitable enough to sometimes commune SSPXers.)

I'm sure your SSPX pastor doesn't literally mean "don't worry about it."  If you hadn't screwed up, and ahead of time you told him you were gonna miss Ascension Thursday, I'm sure he would've encouraged you to go.  

But at the end of the day, its only the Church that decides if these things bind on pain of mortal sin, not the SSPX.  The thing is the SSPV *literally* thinks the post conciliar claimants have no authority, so obviously they can't change the day.  But BOTH SSPX and SSPV would've had to admit it wasn't a mortal sin if Pius XII had made this allowance, however imprudent they might've thought they were.

In other words the application is downstream from whether one thinks Francis is merely a harmful pope or whether he is no pope at all.  Non Sedevacantists who think that you can say its a mortal sin to follow the modified calendar are honestly kidding themselves.  I'll be blunt and take the downvotes, that's an idiotic position.

SSPX issued 1962 Missal lists The Ascension of our lord as day of obligation. The local SSPX church added an evening Mass for those who work and could not attend in the morning, however I do not recall an announcement on the preceding Sunday that this is indeed a holy day of obligation. 

This is the problem with the one foot in the conciliar church and one foot in Tradition approach.
If the Society would just announce that they are completely in the conciliar fold, it would clarify much, but I doubt they would lose many faithful over it.  

This is why the SSPV's stance makes way more sense.

The OLD SSPX had all the advantages of the sedevacantist position, but without a few of the disadvantages. That's where I'll stay thanks.