TomD
1) So since you just got this from parts unknown, my slow cooking method would be to set it aside for a week or two until it has time to settle into your shop environment. But I asume you are hot to trot, and if the dai has straight grain, worst case you will need to work it a little later.

2) There are two type of twist. In one three points are coplanar, and the fourth is above the plane, in the other it is below. Given you want to leave the mouth undisturbed, the first is the easiest, arguably the second you would build up the dai for.

3) Is the blade firmly fitted with even pressure. If when seated it does not sit level, you essentially have a a 5th point kind of situation.

4) If you don't want to mess up the mouth, you don't want to plane it.

5) There is no reason to plane it level fore and aft; those surface aren't coplanar. And the aft surface does not even need to be twist free relative to the front.

6) I would actually start with the idea of where the waves would be, and I would relieve those if you want to plane something. For fun, I plowed these out on a router table once, when I didn't have a scraper plane I liked. A hand scraper works also. If this plane is not a jointer, then the only areas that need to be coplanar are about 1/2" ahead of the blade, and at the toe. And the best way to get those coplanar is to use the sand paper method.

7) if the twist consists of one point low, you can build if up with epoxy, or a shim of wood first.