I guess most people just don't worry about the brakes that much!
On a serious note, looks to me like the same setup the rancher 2 had, in that the brake disk just rides against the frame side and the arm side pushes against it. there is a set screw in the brake disk so it won't float like you would think.
Believe me, brake disks are a problem on greenfield just as much as most rovers, I've cut them off before. That one doesn't look in bad nick, so with a bit of heat and some careful prying you may get it loose.
You're other option might be to space the frame side pad out so it almost touches the disk, then it should all work as intended i think.
I have had trouble with the rivets in the aftermarket pad kits too, the rivet just pulls through the factory hole. You could use a bolt, but the idea of an alloy rivet is as the pad wears away if the rivet hits the disc its no big deal, whereas steel on steel will damage the disc. I would see if you can find some bigger alloy rivets
