Sounds like the LBS has the right idea. Let the fork soak for a couple of days with repeated applications of penetrating oil. If the fork is still stuck take block of wood and hole it on top of the fork and hit it with a hammer good and hard. May take a couple of blows but with a good soaking it “should work. Next option is to cut the steerer tube close to the head set and then cut notches in the remaining part of the steerer. Good luck. Keep us informed of the outcome.