I agree, that would be quite skilled for a hobbyist.
How can one tell whether a piece is cast or not?
Lots of odd shapes are made for engravers by various companies. Some hobbyists can have very highly developed skills, just no interest in trying to make a living - so they can spend a lot more time than a pro and achieve the finest quality. I was puzzled by a signed glass by a known Studio as it had excellent engraving but I had not known they did such work on their non-art production! On asking them, they said that one of their customers must have done the engraving and they had no idea who.
Cast and hand finished does not eliminate the fire surface of casting, although they sometimes get sandblasted to conceal that. Usually the hand finishing is just to add or refine details... enough to add the cachet of hand-made. I don't knock it as a second quality though, as it allows local glass workers/companies to compete in the gift/collectable trade.
But if the mouse is seen in multiples, then it is more likely to be a company or professional engraver.