I'm asking you for a coherent theory for how it all works out, and how that would be worked into a future expansion. I'm asking you for a concept we can tangibly discuss.
I am not asking or expecting you to improvise answers for every issue I bring up. My questions are intended to open you up to asking yourself these questions, and furthering the concept as a cohesive theory. As I said, I'm open to discussion if we have something to discuss.
Right now, all the answers you have are taking us further and further away from your original concept of a Dark Ranger/Night Warrior/Dark Warden hybrid class. Mawsworn Kyrian serving Sylvanas to raise new Undead has become a completely disconnected idea to your original gameplay concept. I don't see how any of this is actually connected together; you're merely improvising answers without considering what the concept is actually supposed to be about.
Like for example; During BFA, we could theorize that Sylvanas could have introduced Dark Rangers into 9.0 because BFA opened up the precedent for Dark Rangers on both Alliance and Horde. How does it work? Well she used her Val'kyr to raise Forsaken Night Elf Sentinels into her service; Rangers who have already been trained in the use of Bows and ranged weapons. Their powers could be explained to be themed around Banshee powers similar to Sylvanas, with abilities like Wailing Arrow sourcing from their inner torment. The Dark Warden Sira Moonwarden could teach them to summon the powerful Avatar of Vengeance, calling forth Spirits of Vengeance to fully express their hatred of the Living. This would be a concept for Dark Rangers using what we knew of BFA lore; before Shadowlands came out. And all of this works, because the lore as ambiguous enough to allow this to be speculatively plausible.
Of course, Shadowlands told a very different story, and many of the concepts in this theory simply do not work today. She no longer has Val'kyr. Hunters are shown to have direct access to abilities like Wailing Arrow and Withering fire, meaning these abilities are
no longer exclusive to the DR identity. Sira Moonwarden was a bloodthirsty renegade and clearly not a Sylvanas loyalist; she has no interest in the Dark Ranger group at all. Most of these ideas are left at a narrative dead end, which renders this theory moot if we consider current lore.
And to address your own concept; it's quite possible but it has no cohesive sense.
Take some time to actually think about how the concept would work. Treat it as if you were explaining the Starting Zone narrative for a Dark Ranger class.