@Hank
While I get your point about some Black magic users being against Outsiders, we have yet to see any evidence of that in the novels. Sure, there are users like the Korean kid in PG who don’t overtly support or have any connections to Outsiders, but there is no evidence that they are opposed to them. Nicodemus is not a wizard, in fact the Denarians with abilities to wield magic are Thorned Namshiel (on Black Council) & Tessa (also likely Black Council or being used by them).
The point in the series is that most wizards will use a little black magic in their time... the world of the DF almost makes it inevitable - the ones who keep using it without caution almost always become corrupted by it. My WAG is simply about the ones who have become corrupted by it - I think that taint opens up a ‘gate’ of sorts to the Outside (& may even be one of the requirements to open the Gates). If you think about the Laws of Magic, doesn’t the 7th one about the Outside seem like a non-sequitur?
I don’t think there is anything normal in most uses of black magic - they are all somewhat unique in manifestation, motivation & abuse (see Molly vs. beheaded Korean kid).
Regarding your point about the 2 concurrent conflicts - hmm that’s how most of Harry’s casefiles end - a couple of seemingly disparate things going on that end up being intertwined in the end (Blood Rites included).
I believe others have already commented on how the Swords can do a lot of other things in addition to fighting the Denarians. The purpose of the Swords is to fight evil of any kind. The purpose of the KotC is the fight/ save the Denarians. The KotC do not need to dedicate themselves to fighting Outsiders because, as you point out, it is Mab’s job (presumably given to her by TWG or his agents).
You're wrong on both counts.
1.) All the denarians use magic, though some are certainly more proficient than others. Nicodemus' shadow leaping off the wall to strangle people is definitely a use of magic to kill. The same goes for Quintus Cassius, who was one of Nic's liutenants and explicitly a magic user with his snake-intensive spells.
2.) In Cold Days, Harry tries to get Murphy to bring a Sword, who refuses, saying explicitly "This isn't their fight". Also, if you think the use of Swords against the Ramps has anything to do with the outsiders, I encourage you to re-read Changes. It's
explicitly stated that they're being punished for being "false gods".
3.) I'll add another point. Outsiders cooperate, they do not fight. The direct quote is "Outsiders; they work together, all of them". This means that, in your black magic=outsider influence model, all properly corrupted black magic users should be working together. Cowl, who is clearly aligned with the outsiders, was competing against and fighting Grevane and Corpsetaker, who are
very obviously fully corrupted by black magic. It's obvious that they are not working with outsiders, or they
would be cooperating 100% with Cowl.