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Stillson sounds like the best idea to me or the tap around the rim with a sharp screwdriver or chisel .
While I fully understand what you mean by chiseling it off and agree it is the best method, I am sure most people would not understand until shown how to do it.I did suggest the chisel early on, and this getting this off has been going on far too long now.
Is it off OP?
As mentioned always use the new O ring and it will not leak, and you never need to pull it down tight.
Just as it starts to nip is fine.
But of course check check check regularly.