I agree with almost everything you’ve found, but I think saying that Daesh is just a bastardization of Islam is a bit of an oversimplification.
From what I can tell, they are Salafi in movement, Athari in “creed”, and de facto Hanbali in madhhab. This is very similar to Saudi Arabia for example (though I realize Ash’ariyyat is taught there).
But what I’m getting at is that if I were a Salafi anti-theology Athari and de facto Hanbali, I’m not sure how you’d convince me that Daesh isn’t legitimate Islam except maybe to point to the illegitimacy of their “caliph” (who doesn’t meet the requirements) and invoke some principles of fiqh like maslahha that they seem to be ignoring (while they can quote Ahmad Ibn Hanbal, it seems like they mostly decide what they want to do then come up with a post hoc justification, not to mention the rather brutal, zealous, and unsophisticated enforcement of sharia law).
While the majority of Muslims in the world aren’t fooled by their fraudulent “caliph” and claims to have a caliphate, some are and it seems that more specific delegitimization efforts are needed. I’m sure these detailed efforts are being made somewhere but it’s hard to find the details of such efforts.
Explaining these details to non-Muslims could also be useful since treating everyone like a simpleton by trying to claim “nothing to do with Islam” is not working out too well. The only real option is to try to explain how Daesh diverges from mainstream Sunni Islam.