The vast majority of monsters are not immune to gunpowder. that's a fact. The vast number by type are also not going to USE gunpowder, unless you're playing a primary-humanoids campaigns. In short, you're now talking about changing the monsters so gunpowder is useless as a fix...
No, but there are a lot that are resistent/immune to fire/nonmagical damage, others with craptons of hp, etc. Most monsters don't need gunpowder—they have other abilities.
Then suddenly this stuff they jump through hoops being able to make is now available to creatures that live at the level of medieval vikings or Native Americans before the Europeans came. Okie-dokie. Instant civilizing and educational systems, OR, suddenly this hard to make and procure stuff is now being sold to the enemies of the PC's, and all humanity, by unknown parties.
Well, if you introduce gunpowder to your setting and it becomes as common as you suggest, then those creature "that live at the level of medieval vikings or Native Americans before the Europeans came" (which is kinda an icky statement, BTW) will adapt to it (just like Native Americans did). Sounds like your humanoids are very two dimensional if they are the enemies of all humanity. Even then, "unknown parties" have a long history of selling arms to the enemies of their enemies in the real world. BTW, is it hard to procure or commonly available in you scenario—I can't keep up?
So, these examples aren't selling it to me.
Your scenarios aren't selling me, either. So, I guess we're at an impasse.
The supplies are, by default, assumed to be available in most larger settlements, and are certainly within the price range of them.
Wait, I thought that they were hard to procure. So, even if available in most large settlements, in what quantity is it available—to the vendors need to resupply their stocks or does the gunpowder and its ingredient just magically resupply after the PCs break the settlements economy (again).
Your restricting that supply is now making house rules. Just be aware you are doing it.
Given that gunpowder avaiability is not a default in D&D, determining how available it (or anything else) is is not a house rule. Different settlements, cultures, and locations will have different resources.
Gunpowder hoarders are far less dangerous then fireball casting wizards walking around the streets, but if you want to be DM Arbitrary about the threat levels, go right ahead.
Oh? That seems rather counter to your previous assertions.
You need one rank to assist in helping with alchemy in 3e/Pathfinder. NPC's can do most of the work. Referring to 3e rules, higher skill targets for the DC enable you to make more stuff. There are also magical tools that greatly accelerate making alchemical items, as well as that old game breaker, Fabricate.
Well, in 3e, who needs gunpowder when you can be CoDzilla? Oh no, there goes Tokyo! Thankfully, I no longer play 3e or PF. I'm not going to bother looking up the 3e SRD to see the specifics of fabricate, but in 5e it would require proficiency in the appropriate artisan's tools (which the DM will need to decide if it uses alchemist's tools or a new "powdermaker's tools" or some such), and still require the raw material that would otherwise be needed to make it. It's also a 4th-level spell, so there is another resource cost.
As Create Wondrous Items is one of the most popular of feats, making your own tools isn't hard, and they pay for themselves VERY quickly. Artificially not letting them is again a DM Fiat ruling.
CoDzilla. But Create Wonderous Items is used to create magical items, has a cost in both mundane resouces and XP, and you could use it to create more interesting things.
Alchemist Classes also generally can make alchemical items far quicker than normal people, too. They usually have it baked right into the class.
If the rules you use have an alchemict class (I guess that's a PF thing). Again, depending on the rules you use, alchemy way not even be what is used to create gunpowder.