Is having the tailplane at the wings backwash a design flaw?
I would say not really. The main impact would be on what incidence angle to set for the horizontal stabilizer because of the difference in local flow angle in or out of the downwash, and on the trim changes to expect if there are wing flaps, larger trim changes in the downwash, less if out.

For your ultralight, I would say the priority is simplicity and lightness, so just stick the horizontal tail at the most structurally convenient location, which is more or less where it is in the picture, like 90% of the other airplanes out there.  

Size it based on the standard formula, make an educated guess (er... calculation) on the incidence angle to set, but make an attachment for the stab leading edge with multiple holes that allows you to change the incidence after flight testing.  

And remember that when you test fly your ultralight you are effectively an Experimental Test Pilot taking the risks that go with that job description.