Nice! That rust is pretty common on 240s. Possible causes are windscreen leaks, wiper pivot leaks and blocked A/C evaporator drain - all allow water to leak in under the carpet unnoticed, which usually finds its way back to the rear floor area. It sits there in the foam insulation and never dries out...
Never saw the rear seat pivot brackets break off before, but I guess if it was used for heavy loads often with the seat folded down, it could have knocked them about.
Looking forward to watching the build progress. Looks like you put those used struts to good use! :)