Yup, that's what I'm getting at. This base level connection that the val'kyr have to Sylvanas is also why the 'berserk state' Forsaken raised default to loyalty to Sylvanas, because in that window of time where they lack conscious will it's substituted with the will of the one who they're connected to, being Sylvanas. Once that wears off and their personality surfaces, they can make a call, but if you really want to push it they're more susceptible to suggestion still.
The rest of this theory is interesting as well, though it does get weird in Sylvanas' case unless we assume she's tied to the LK or Yoggy. The alternative is that the val'kyr are expanding her own abilities, but are still tied to them. Hence why the LK-aligned val'kyr could do a lot more, since they were tied to a more powerful figure, whereas Sylvanas was initially just an average necromancer and could only raise a few things. As she became more powerful, so too did they with them feeding into each other.
All speculation, but it's fun to consider.