Ken,
I do not understand your statement that "if you don't have a strong center section and spars that can flex some none of the force on the center joints is absorbed. " Force isn't really absorbed unless you have damping, or a shock absorber, or.... Forces get transmitted from one place to another, or to a group of places. This path is called the loadpath. This may be the single most important thing to understand when you are designing structures. In general, all the weight of the fuselage (and everything it contains) is reacted into the wing.
Look at the load case from a hard pull-up. You get a lift distribution spanwise, so the load is not constant. But no matter what (except in planes with struts) you get the maximum bending load (moment load) at the wing root. If the fuse/joint is stiff enough, you could argue that some of that moment gets reacted by the fuse sides, but you'd be better off neglecting that in any calculations. So the max bending load, which needs to be taken up by the spars and the planking happens right in the middle. No surprise there.
What you do not want is a discontinuity in the stiffness/strength of the wing structure. This is a stress concentration, and that usually means it is going to break there. How many times have you seen a wing fail right where the center planking quits?
X-braces attempt to accomplish the same thing as a shear web. They just aren't as efficient in my opinion since the brace in compression will attempt to buckle. So by all means, glue the braces together where they cross. Next, the joint geometry and glue joints need to be good. Pretty common to see the braces (and shear webs too) scabbed on the trailing edge of the spars instead of a web like in a steel I-beam. Not the best structurally, but easier to do. Most guys find one way of stiffening up the spar system more suitable for their tools, methods, skills. And so it works best for them. As long as there is positive margin in the overall construction, you're good. But using the optimal geometry lets you shave a tiny bit of weight. Maybe not too important in a stunt plane--but crucial in a full-sized one.
Dave