Well, from what I have seen those wings that do not use fins usually have wingtip angled leading edge to counter sliding. Others (B-2) use aileron brakes to have directional control.
Something has to be done. Some ideas work better, the others' are not so good. Aileron brakes introduce an extra drug. I'd go for fins on RC model since we're not after stealth technology and fins work just as good on slow and high speeds.
Birds use their tail by twisting it as well as wingtip feathers as a fins but they have software far more advanced than we have to control the airframe.
I was about to propose the swept wing, but I like your design in general.
As far as spar goes - what's the expected weight of the plane? My guess is about 800 grams ~(27 once) all up? You can use one 90 degree molded carbon fiber spar for that. That way it would be very light and strong. Foam will hold it against the twist load. Or you can buy a square box tubing, cut two identical halfs and then do the exact fitting on each of them, or just use one if it's long enough.
Infinite solutions as far as design goes.
just a few outloud thoughts.
cheers,
Mark