A bit off topic but why would a red car hurt resale value?
I think it is a good idea to mod the M3 vs. getting the CTS-V. But I think the way to go should be to make a CTS-V more like an M3 rather than the other way around. LOL. Sorry, I can't really answer your questions. I'd think the fixed seat back is a good idea and it seems it will fit your ideas too but I'd try sitting in it first if you can.
I think your issues would not be in installing a fixed seat-back, but in installing an aftermarket seat. If I'm not mistaken you have to fiddle with the electronics to keep warnings from going off. There are solutions out there so I'll let other more knowledgable members chime in here. Good luck.
I've been assimilated. 6MT/ZPP/18"/PS extended.