If the spindle won't turn independently of the derailleur body, how did you get it on/off? I must not be understanding your issue.

Shimano specs torque at 8-10 Nm, so it shouldn't have been that tight unless maybe the threads are a little corroded? How does your derailleur hanger look? Maybe put a little anti-seize on the threads when you reinstall it ...

I've never tried to disassemble that part of a derailleur. I just put a drop of oil in there (and all the other pivot points on derailleurs and brakes) after I wash my bikes. Exploded diagrams are here - you didn't say which series your derailleur is. There are no springs on the spindle, but you'd need a cerclip tool.