All front wall: definitely, yes. You're not supposed to get sound from the front that has come from side, rear or atmos speakers.
Back wall: It's not so much 'careful', as the fact that a bit of reflected sound off the back wall from the side surrounds etc can help create spaciousness.
How would I feel - am I moving in?
Maybe just start with half of that, in the centre, and see how you go? But if you did do 50-60%, it's not going to be awful. But less could be better, and some diffusion instead of all absorption would almost certainly be better.
No, sorry, but I'm sure others will. I'll make my own. I believe you really want them quite deep (more than 10cm) to work well (low frequencies).
Do you have a blueprint you will follow? With my new toy (mitre saw) I could easily create few from wood. They will weight a tone but should do the job.
EDIT: Ok, found something like this which would work well with the tools I have: How to Build an Acoustic Diffuser — And Why You Need Diffusion – Soundfly
Well it's the centre channel that benefit most from being behind a screen. It wouldn't matter if your L&R are to the sides of the screen (it does depend what angle that would create at the listening position - something from 45 to 60 degrees would be good).
If you can benefit from your towers elsewhere, then yeah, get 3 matching speakers behind the screen. But if you don't need the towers elsewhere, or budget doesn't allow, then the towers either side with something with as close a sound to them as possible in the centre would be good. Please don't have a horizontal speaker behind an AT screen (although it can be fine if you watch films on your own).
Now, you have thrown a spanner into my plans . Putting C behind AT sounds like a nice middle ground solution. I have to take some distance measurements in how it would impact seating position, max zoom of PJ etc. and I would have to find new AT screen but that would be great option. Also allowing me to easily add L + R behind the screen in the future if I feel like it. Just makes the whole operation a bigger job. But probably well worth it.