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The weakness/resistance rules are a bit ambiguous, and in running this AP it's far more relevant than it's ever been before as almost every demon is affected. I've hunted for answers, failed to come up with good results, and a lot of it seems to depend on a several year old forum comment from Mark Seifter which seems not great as a definitive rules source. Part of the reason this is an issue after all is because of how the remaster changed the rules on a few things, and that long predates the remaster.
The bad news is that there has never been any clarification on this beyond what Mark gave us, and as Mark's name is on the cover of the CRB, he probably has a decent idea of what he's talking about.
People have been asking about this for years and it's never been fully clarified. So if you want an "official" answer, you're out of luck: there isn't one. I don't know why they refuse to clarify something that so many people find confusing, but that's what it is.
Given that, I generally use a "do what Foundry does" philosophy. That's both because I use Foundry and doing anything else would be more work, but also because it's popular enough for PF2 games that it's the closest thing to a community standard we have. That doesn't mean Foundry is actually doing it correctly. It just means that enough people are using Foundry and thus doing it that way that I can show up at a table and people generally know what to expect.
(Yes, that does get quite messy with the Mythic gives "Weakness All" ability, but it does mean Weakness All and Resistance All work consistently, so at least it has that going for it.)
The leading consensus is that if an effect deals multiple damage types, these should each be treated as a separate instance of damage and weaknesses/resistances should be applied separately. This post is mostly made because this seems to present issues to me, particularly in the case of demons and holy weaknesses which is extremely relevant to this AP.
Yes, doing it this way does mean a lot of weakness if you equip yourself for it. It's definitely going to make Spore War easier with how often it comes up. And yes, that is generally how it gets applied (that's how Foundry does it).
There is also the rule about non-damaging weaknesses, which can be used to make the argument that if you are damaged by an effect with the water trait, holy trait, or something else not associated with a specific damage type you apply that weakness on top of any that would apply from the damage itself as it counts as being touched or affected by something with that trait.
This is still one instance of damage, just with multiple traits. So if you have a Cold Iron attack and you add the Water trait, but not water damage, you're not hitting both weaknesses (just the larger one).
Now, Demons have a Vulnerability that isn't a weakness, so that is handled separately and if you trigger that, it would happen regardless.
My personal reading, just off the rules in the book, was that one strike/impulse/etc. is one instance of damage, and holy modifies the damage similar to cold iron, so every single scenario above the answer is only the highest applicable weakness is applied. Even in scenario 9, my interpretation was that the non-damaging weakness rule is probably not meant to apply to double up on weaknesses if damaging effects have the relevant trait. I'm not sure this is correct though, and it's been an on and off topic due to Spore War for us for months now.
Like I said earlier: no one actually knows if it's correct or not. I will say that doing it the way you are mentioning here has some side effects in other areas. Do you treat resistances the same way?
ie: if something is resistant to slashing and acid, does a corrosive shortsword hit both resistances or just one? By your ruling it should only hit one. By the "foundry ruling" it hits both. If it only hits one, that means that having multiple resistances works differently than "Resist All", because Resist All is called out specifically in the rulebook as applying to every part of the damage on the attack and it explicitly can apply more than once (ie: it would apply to both the slashing and the acid).
So if you do the whole attack as one instance of damage, then Resist All is different than "Resist to every type of damage this attack is doing", which to me doesn't make any sense and feels like an awkward exception. Whereas if you treat them as different instances of damage, resist all/resist multiple work exactly the same way and so do the weaknesses, and thus one method of resolving it covers everything.