I like the center section as part of the airframe that will save you a ton of weight if you do it right.
Dual tubes yep.
Rather than typical lite ply for the formers I found that 5 sheets of balsa stacked is a lot lighter and stronger, balsa stacked so the two outer pieces are parallel to each other, the next two 45 degrees to those one left one right and the center piece 90 degrees to the two outside pieces (it is also twice the thickness of 4 other pieces). This balsa ply seems to be very strong and lightweight.

With careful design for this size airframe I bet a built up wing could be made much lighter than foam core, for your two outboard panels. Foam isn't all that light and with proper structure all that filler doesn't really buy you much. I would use the stressed skin approach for this aircraft more work yes, but can be done very light were your strength is where it will do the most good, that is where the bending forces are at their max and your further form the center. I look at foam as the easy way out as far as construction so bear with me please.
Also look at your center section wing root and fuselage as a one larger shape and assembly rather than separate components (if you understand what I mean) ...