I have not yet given up, but I have been unsuccessful getting a solid FFing install of the Radian in the center seat -- I suspect due to the forward position of the belt and the fact that this seat does not compress much (because of the factory booster seat).
I am able to get the Radian installed in the outboard FFing position with LATCH. It takes a some patience to get the straps tight (pulling the loose end through one of the Radian's velcro access slots makes all the difference).
Regarding the headrest... I have tried the install two ways:
1. Place the top edge of the Radian just under the headrest and then slide the base towards the seat back. This will compress the Radian into the seat and allows for a fairly solid install.
2. Place the top edge of the Radian over the top edge of the headrest. When tightening the LATCH straps, use down and backward pressure to get the Radian seat back against the Volvo's seat.
After playing with both installs, I like #2 as it yields better use of the top tether. In install #1 above, the tether strap would bind on the seat, making it difficult to tighten the strap (in install #1, the adjusting top-tether buckle falls in an awkward angle).
I still have not given up on a center install, and plan to attempt again using some of the tips and tricks seen
here.