It sounds like the retaining ring got cross threaded (possibly there were uncut threads nearer the shutter flange. This could have happened if the lens board is thin enough to allow the retaining ring to partially move into the uncut region. If I were you, I would try to screw the ring back onto the lens...depending on how much pressure you have exerted to this point, it may be resistant. If you can do this, possibly you may be able to straighten the orientation of the ring threads to the shutter threads.

I would next probably look to enlisting a local machine shop or even SK Grimes and get the lens to them for removal of the ring. The cost may be less expensive then a replacement shutter.

Barring that, cutting the lens board would accomplish nothing insofar as your stuck ring (it would still be stuck). I would probably then try a dremel tool and very carefully work at cutting the ring. The reason that I stress carefully, is that if you get into the threads on the shutter, you can kiss it good bye.

Good luck.