Great question on hinging the trailing edge between the rudders.

In forward motion, air is traviling across the leading edge and towards the tip of the wing. This flow, or displacement of air, alows these control surfaces to be effective. When hovering the only air moving over the control surfaces is created from the prop wash. This wash has the strongest effect from the root of the wing spreading outward. This is why designers have moved the root of the ailerons closer towards the fuselage placing them in the prop wash and therefore more control for 3D maneuvers.

By hinging this center section you will see an increase in your elevator control and the twin rudders are already in the prop wash, so this leaves the aileron function of the elevons. The ailerons will receive some propwash, although not much and if you split this center elevator in two thus creating a mini elevon, the effect would be minimal at best since the control surface is right at the root of the wing.

Retracts, if designed and installed correctly will work great off of grass runways. Look at all the warbirds, nitro through giant scale that fly off of grass runways. The biggest problem I have seen is ARF manufactures skimping on the structural support, and the use of hot glue instead of epoxy. A well designed retract rail supported by ply doublers will displace the loads incurred from a small area out over a larger one.

They are hard to see in the pic below but both sides of ribs W-1, W-2 and W-3 have 1-8" ply doublers, thus spreading the load. Ultimatly the test is in the landings and while I have had my share of hard landings, I have never torn, ripped or pushed a retract from it's mount. I have BENT a few wire struts. LOL!

Hope this helps?

Mike