> but do we have the facings of all those 26 options?
Yes.
They can all be derived from that 3x3x3 grid, on condition ofcourse that you don't hollow out the voxel object, before you apply the normals.

> Still having some issues with 1-layer thick voxels though (because there are a lot of them).

I don't see much use for 1 layer thick voxels, unless you hollow out the voxels.
Note that most of the WW voxels go around that problem by using a rather compact volume for the voxelmodel. You rarely see a one layer thick voxel layer (vertically or horizontally).
When making a voxel model, one should be aware of the limitations of the normals, so if you need a double sided thin layer, you'll HAVE to make it 2 voxels thick to give the layer a front and back...

but that ofcourse doesn't solve the problem when you do have a 1 layer situation.... it needs more pondering.. ;)