Thanks for the well documented story!

I've pulled my Cyclones apart for general maintenance and to replace a broken spring. As noted, there are a couple of challenges in the process.
My solution to unthreading the axle/bolt from the pulley cage plate is to slide the cage/bolt assembly back out of the derailleur housing, which exposes the bolt enough to just grab it with a vise-grips (locking pliers). If nothing else, it provides a bit more leverage than the chainring spanner that you used. There are probably other ways of doing it too.

A quick shot of my disassembled Cyclone, along with the broken spring.



This is the second SunTour spring I've had break. The other is a somewhat similar coil spring on the XCD Pro cantilever brake. It also broke at the bend where the spring was supposed to engage with a locating hole.

Steve in Peoria