The rubber in the seat valve dry rotted and failed...
There's a small relief point in the diaphram that normally should pop or a relief valve on the side of the regulator body. In your case it sounds like you had a relief valve and it also siezed. Very rare to see a regulator bell blow off like that.
As mentioned before, no danger with inert gases and
oil, just oxygen.
I used to repair welding equipment many years ago.