This is all true and I'm not trying to get you to go one way or the other, but if you think you might benefit at all from the added roll stability dihedral gives you, consider that 10 degrees of dihedral per panel (which is a lot) only decreases the effective span for each panel by 1.52%. 5 degrees per panel only costs you 0.38%. You can gain significant additional roll stability and lose almost nothing by using a few degrees of dihedral. If it were my project, the tiny amount of lift loss by itselft would not be a deterrent to a few degrees of dihedral. You will of course have some dihedral effect by mounting the wing high, but this effect will be relatively weak, considering that your 12 foot wing will only be 8" above the thrust line, and presumably close to the same distance, or maybe less, above the CofG.

Having written all that, I'm sure it will fly just fine with no dihedral; you just might find yourself using a bit more aileron in turbulence.

The only reason I mention this at all, is that I have considerable experience flying (similarly configured) 200 lb, 18 ft and 600 lb, 27 ft, high, flat bottomed wing UAVs that were built with zero dihedral (for simplicity and efficiency), and I wished they had been built with some dihedral because I had to fly them for long periods in turbulence, and the additional drag from the additional aileron inputs easily ate up any tiny efficiency gain they had from zero dihedral, not to to mention it got a little annoying at times. Even though they had high wings (there was no "cabin" so the wings were mounted on top of the box fuselage), their roll stability was so weak that it was essentially ineffective. In short, they would have been more pleasant to fly with some dihedral.

Just curious: Why split flaps? Do you plan to use them as brakes to get down faster for the next tow? If so, that makes good sense.