If Reinhard is the direct accomplice/subordinate for the witch, then that can be worked around.

I will be somewhat disappointed if "mysterious witch we haven't seen" is really the mastermind. If there even is a mastermind. Maybe I'm just giving the story too much credit.

There are a lot of plot threads dangling around: someone had to hire Elsa, Felt's background, Roswal, Emilia's background, the witch cult, the Swordmaster, the Dragon, the Witch, etc. I hope they're all at least somewhat connected and don't end up being random/unanswered.

I will reconsider my stance on mysterious witch if she is at least as good as Rem.
If mysterious witch is Emilia, then I think it'll be pretty stupid.

Mysterious witch being Subaru could be interesting, though I'm not sure how you would explain that.