In the case of a daughter bulb, or mobile, or offset , from a modern
hipastrium hybrid, what is its 'level of attachment' to the mother bulb?

A picture might help. But again, as my previous post suggests, if it has
its own basal plate, or is sharing a portion of the mother bulbs' basal
plate, I still think it could be cut free and carefully cultivated with an
excellent chance of producing a healthy bulb.

There are members of PBS that have extensive hip-hatrium experience and
propagation, and study on a university level. Perhaps they will jump in
here.

Ken