Wyrm, from the incident with Onni's ear it looks as though Minna is using the same rules as apply in many our-world magical traditions: damage to the spirit is reflected in the flesh. For example, someone drowning in the dreamworld might choke on their spit because they are unconscious and haven't been put in the 'recovery position' that one learns about in first aid courses, or their house might flood, or they might fall asleep in the bath and drown.
Cracking both the air bubble and the shield might do similar things, or induce the breaking of bones from some 'natural' cause, or induce a stroke?