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I figured the laws about the location are more disciplinary, while the requirement for a consecrated altar is more of a necessity to preserve the sanctity of the Sacrifice of the Mass itself. Those probably aren't the most proper terms, but you get what I'm saying
In my opinion the  garage or hotel masses display the reality of the crisis in the Church much more clearly than certain trad groups like the sspx building massive chapels and calling them churches, with parishes, calling priests pastors. I think that can cause the laity to become lukewarm, to assume that things are pretty normal and that we just have some not-great popes..rather than that we are living through the great apostasy

So, both do pertain to the "sanctity" and dignity of the Holy Sacrifice.  Generally speaking, the use of an altar stone was seen as somehow mitigating the surrounding environment, but it did not completely offset it, or you could then put an altar stone anywhere, including in the middle of a brothel, right?  In fact, you'd probably be committing a second sin by profaning the altar stone by putting it in such a setting.  But the principle behind both considerations is the same, that sanctity or dignity of the Mass.  And you couldn't just say Mass anywhere in normal times as if the Altar Stone took care of the problem entirely, but required the Ordinary's permission to do so.  In other words, you required the permission of the Ordinary even to use an Altar Stone in a certain unbecoming setting ... for various pastoral reasons.  But the same dispensation could be granted even to not having an Altar Stone, again, for the good of souls, such as where you might secretly have Mass in some cσncєnтrαтισn cαмρ on a table with obviously no altar stone available.  In other words, the rule is that it should be in a sacred space consecrated for the Mass ... but if there's some urgent pastoral necessity, elsewhere, and then if it's elsewhere then the priest should use an altar stone, if possible, but that too can be dispensed with for some urgent pastoral necessity.  It's not like one is an absolute requirement the other not.  Both are already exceptions to the rule.

Maybe a good analogy is this.  Let's say a priest needs to offer Mass (for souls) but doesn't have vestments.  Well, the rule might then be ... if the priest has to say Mass without vestments, he should take care to wear a suit (vs. some slovenly outfit like jeans and T-shirt) ... but then if that's not possible, for urgent need, he can offer Mass with jeans and T-shirt (it's all he has, it's in time of persecution, souls need the Mass and Sacraments, etc.).  That's the analogy here.

In terms of "hotel rooms", there's a distinction between a rented hall, for instance, and a priest's own hotel room, IMO, where the the former addresses a pastoral need, whereas the latter might be one of convenience.

I think so.  There's just so much confusion.  It's not like the SSPX priest is going into the NO Church thinking he's profaning the Mass, as if he were, say, entering a ѕуηαgσgυє or mosque.  Some churches were even consecrated by valid bishops before Vatican II, but even the ones that weren't they still professed Cathoicism (vs. those dedicated to professed non-Catholic religions).  So, another thing to consider is that if the church is suitable for a Tridentine Mass, it likely still has the old high altar there, which likely was not use for the NOM (vs. the Luther table in front of it) ... distancing the NO altar a bit from the true one.  In any case, while I personally consider the NOM objectively sacrilegous, not everyone shares that view, thinking it's more defective, deficient, lacking, ambiguous, or mostly bad because of the various implementations of it.  While I don't share that view, I also realize that it's only my own opinion.  I would not go to such a Mass if I had any viable alternaties, but if it's all I had to fulfill my Sunday obligation and receive Holy Communion, I would go ... due to the objective uncertainty.  I liken it to a question of moral theology where priests might have different opinions.  One priest might say something is OK, whereas another might say it's not.  So what do you do as a member of the lay faithful?  Well, moralists generally hold that you'd be permitted to go with either opinion provided that the Church hasn't rule on the dispute against one of the priests's opinons.

It's generally my opinion that there would have to be a very high degree of certainty about some matter that all Traditional Catholics agree upon before the lay faithful would be required in conscience to refrain from assisting at Holy Mass.

Thanks for the explanations, on both counts. That makes a lot of sense. From a different angle, another reason the SSPX using NO churches bothers me is that it gives the NO bishops a semblance of holding lawful authority in the Catholic Church, and how they are charitable, merciful in deigning to allow a "canonically irregular" group to celebrate the true Mass in their churches. Now obviously a Catholic bishop does have the right to do that, and I guess the sspx has always held those bishops to still have authority..but it just another one of those things that I think can cause trad faithful to become complacent, and believe the situation isn't as bad as it really is