I'd go with the least expensive option first.... Insert a drill bit slightly smaller then the tenon and wiggle it out from the shank. Maybe even a 1/4" flat head screwdriver, to ease or spin it out......gently.
Apply crazy glue to the stem and tenon.
When in doubt, Mac Gyver it out...
(as long as you don't create more damage?)
Absolutely not a good solution NOR a proper way to go about solving any problem of a similar kind. (Because damage only becomes known after the fact, and there's no rewind button.)
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Regarding the new tenon or new stem question, neither solution is simple. A new stem because making stems for existing stummels is MUCH more labor intensive than making them originally, and jacking the stummel in the process is not uncommon (again, there is no rewind button). Even the original makers have trouble with this. I've seen Castello (as well as others) put out some embarassingly bad repairs over the years.
Replacing tenons is straightforward in concept, but fiendishly difficult to do well. Sloppy work is more common than good work, by far. "Functional repair" is usually the standard, not "invisible" repair.
Prime Castellos fall into the zone between tobacco access device and valuable collectable, so the direction to take in this case isn't clear. If you just want to smoke the pipe again and don't care how it LOOKS, that's one thing; if returning it to original form/condition is a requirement, shop around carefully.
