Oh yah, just saw the sandbags now, the mg does seem to be floating a bit, but that's easily remidied, take some tissue or milliput and make a small tarp to stick under the mg to fill the gap, and maybe add some rope to the sandbags to help "tie them down", if the driver suddenly hit the brakes, they would shoot forward.
As for the decals, probably not much you can do now, unless you scrape them off and reapply, but that would kill your paint. For future, when you go to apply decals, I usually just brush on a coulple coats of Future Floor polish where the decal would go, when dry, apply the decal. Then you can add your filters to help blend in the cammo and decals to the vehicle, then add a couple coats of flat lacquer to finish. Good use of the old Tamiya figures!
"There you go with those negative waves again!"