@Madeleine52, sorry you have been victimized by this bad person!
Whenever someone books our listing, I at the very least copy what little information AirBnB provides about the guest. (Their full name and phone number). (Once the stay is over, you lose access to the phone number). If any other information is discovered later, I also write it down.
This should at least help with finding them later.
You mentioned that 'Doris' texted you; do you still have the text? Would that at least provide her phone number?
I know you are upset at AirBnB for not being more cooperative, but mainly you want to find 'Doris', who is the actual source of your pain.
(Side question: I don't understand how you feel AirBnB is encouraging thieves.)
I hope you find this bad person, and I hope your assistant does not lose their stuff any more!