Thoughts?

By definition, all rebellion against the empire is heresy, so yes.

Is the Severan Dominate Chaos-tainted? No.

Certainly not to any degree larger than the regular empire, at least in the beginning. As things go on, probably more because the anti-Chaos institutions can't do their work properly.

Is the leadership tainted? No, not as far as we know, one hint (which might still be DE-related) notwithstanding.

They are by definition heretics because they are rebelling against the rule of the Imperium.

I think it's hinted that they're backed by the DE without their knowledge, so that just makes it doubly so. Although I think that's a cop out.

As secessionists , not as apostates . Officially it's a heresy, but then, "what isn't?"

That is, of course for the loyalists they have abandoned the Emperor's way because they ditched Imperium. But the Duke's followers say they ditched the Imperium as corrupt, but still have faith in the Emperor.

In most ways they are no different from Imperial people and most won't deal with DEldar, though apparently a few high-ups are desperate enough for that.

Yes, I know that I come late.

But to prevent the "everyone is a heretic" label I kind of say that the Dominate are heretics in the sense of them being against the Imperium but since they haven't forsaken the faith in the Emperor, "only" in Imperial institutions I would say that they should be termed "separatists" as they seek to break away from the Imperium.

And from a Severian POV I could see that they can see themselves kind of similar to Sebastian Thor in the wars with, that Ecclesiarch with a name I can't recall at the moment, which lead to a great reformation of the Imperial Church and purging of corruption within it. Even the establishment of the Ordo Hereticus and present day form of the Adepta Sororitas, if I recall. In both cases people loyal to the Emperor saw the rot in the Imperium and refused to serve the corrupt officials while remaining true to the Emperor himself.

What everyone else said lol

It is all about perspective. The important thing here is that they are not chaos-tainted. The everyday SD soldier is still praying to the Emperor, just not bowing down to Imperial taxation/regulation/etc. The Imperium will then go out of its way to label them "hereticus" and launch a crusade to bring them to heel, simply because secession can never be tolerated. First its this sector, then it's another, etc. etc.