I am back on track also. When I saw dihedral brace I thought a few inches into each wing panel and glue. I see that you wish to replace the wing tube with a wing blade. I have never done it, but I see no reason it will not work. You stated 12" into each wing panel, which should work. Obviously the spar box structure must be up to the task as has been stated above. My only concern would be that as the wing is built up wood and the blade is plywood is the fit. The blade must be quite snug into the wing box, since any play will be greatly amplified at the wing tip. What will humidity do to the fit? I live in the high desert where it would probably not be a problem. If you live in an area with huge seasonal humidity changes it might go from kind of sloppy to needing a hammer to take it apart.
Scott