Alex:

Don't think you'll have any problems with missing 3/8" dihedral. I'm pretty sure my wing is about the same, and my US flies fine. I'd rather have my root ribs glued solidly together than have perfect dihedral with a gap in the root ribs.

As for the slight difference in sheeting height, between the fiberglass tape on the middle of the wing, some wood filler, and an 80 grit sanding block, everything will end up smooth and even.