Hi @hazelnot, did you try to call Fairphone? Sometimes this helps if things seem complicated. I think it should be possible to convince them that it is sufficient to tell them the IMEI number of the phone. They have a stolen phone list. They could check if the IMEI of your phone is on that list. If it is not, there is no reason not to help you.
An other option is to ask a Fairphone Angel to help you.