I'm "only" a first year rider, but the experience I do have is with setting up my Flow Quattros and agree it's a little frustrating. Don't give up on your bindings/boots combo, though. I have Motos in 10.5, which I think are pretty bulky and they fit in L bindings fine, even though at first they appeared to be way too big. Looking at your pic, I think the overhang has you trying to move your boot forward in the binding, thereby pushing the straps out to their ends. Have you tried moving your boot back in the binding so it's against the highback and adjusting the heelpad back to it's rear-most position? Moving the boot back would give your straps more room to cinch down and eliminate that space between the boot and highback. Then with the boot mostly dialed in, I think the mounting plate has got to give you enough options to center everything on the board. The Flow YouTube video said that mounting the plate horizontally will give you more options edge to edge, which is where you'd mitigate an overhang problem. Just some thoughts...FWIW...YMMV
Good luck to ya'! Worst case, find a shop kid or someone else well-versed in setting up Flows to help, but I don't think the answer is to return things (if you could).