I took another try at what to do, after looking at the warping a second time. I decided, without a backplate, I would not use the heatsink. Period.
Ok, so I got this kit, which doesn't work with the included kit of screws that are mounted to it. So I pried off the little black tabs that hold the screws into it, and removed the screws and tabs. Having done this, I verified that the screws included with the heatsink would indeed fall through it, if tipped at all. So if you keep them exactly straight they might grab enough to allow the backplate to function.
Next I took the screws and verified they were long enough, after running through the backplate, to reach the thumb nuts on the top of the board. They poked through a good distance in fact, several millimeters. Given that the screws were nice and long, I decided to use the little plastic washers that are included with the heatsink, the ones that the screws normally go through without a backplate.
I then placed those washers on the four screws after feeding them through the backplate. I slipped the washers up far enough to hold the screws straight so they wouldn't fall through the backplate by tipping. The washers catch on the threading of the screws, and are fairly snug, so they don't slip off after shifting them up. Then I put the backplate on the motherboard, making sure each washer's little ridge fit the holes on the motherboard. Once they were all poking through, I attached the nuts, and tightened them on with a screwdriver until tight. Much tighter than you'd make them by hand tightening the thumb nuts. This was to be sure they wouldn't shift and slip through the backplate, as they catch about a hair's diameter on each side of the screws head (putting big metal washers on them would have been nice, but no room as the screws have to poke through the motherboard far enough for the nuts to grab them well).
After attaching the backplate and nuts, I installed the motherboard as normal and added the top plates and attached the motherboard to the case. Mounted the heatsink, and checked the back for warping. None. No warping at all. What a relief.
So the kit worked, sort of, and allowed me to use a Rev. A heatsink with the kits backplate. Feels very secure, even if the screws only grab the backplate by a hair.