CONFIRMED. E90 M3 rear sub frame works with all other E90 spring and damper mounted lower control arm!!
Now I don't have to buy M3 arm and damper to work with it. It would be nice but I already have Bilsteins for the rear. Rear lower forward is all the same, so I have everything M3 control arms except the rear bottom arm.
Corrosion is bad for you, especially brake line nuts. They get stuck on the line, when you twist them lose with flared nut wrench, they twist the line with them and break them. I anticipated this and bought replacement lines that are mounted to sub frame. The lines mounted to the body were broken loose using liquid wrench and propane torch. They were close to fuel tank and EVAP hose so you have to be very careful with torch. I'll apply liquid film to prevent these issues in the future.
Body had some surface rust near rear of fuel tank and took them out. Those are places where snow and salt can get trapped.
Wagon rear brace mount was the last to come out and had the most rust. I guess salt just splatters on it most. I'm brushing out the rust and applying CLR on it before rinsing out and going with Duplicolor self-etching primer, filler primer, black, clear coat, and then Duplicolor Rust Barrier gloss black rubber coat on it. It will have to go in before the rear sub frame. The bolts are virtually inaccessible without removing the rear sub frame.