The mounting on the back of the seats is fine.
As long as they each have their own 4 AWG wire for a ground, that should be just fine. Also, make sure your ground connections are metal-to-metal, no paint. Use a steel bristle brush and clean all the paint off the ground area.
If they're overheating when you're listening to the music quietly, something is wrong. If they're getting warm to the touch when you're listening to quiet music, that's somewhat normal. Amps do make heat, it's just something they do. Heating up to the point of failure though, isn't normal.
Set the gain as best you can. If you don't have an oscilloscope, use your ears and a good testing CD.
With the Type-R's bridged to the 942 at 8 Ohms, that's plenty safe. I'm assuming you've got the DVC 2 Ohm version of the subs? Or did they make a SVC 4 Ohm version?
While a single 4 AWG wire powering both a 564 and a 942 will work, it's on the skimpy side. I'd say run each amp its own 4 AWG wire, that should help out.
Oh yeah... Check
this link out, it should help you quite a bit.