The bootloader was locked, the OEM unlock was activated.
So I was able to unlock the bootloader using Fastboot without activating anything in the developer options.
Regarding the IMEI on Google's website, it finds it for me but doesn't give me too many details:
“Check repair options for the device with this IMEI number or serial number in the hardware warranty center.”
Concerning Auditor it's more annoying. I can check my Pixel 6 under graphene without any problem.
But when I want to check this Pixel 9 Pro XL it puts me :
"Error encountered generating attestation"
"java.security.ProviderException: Failed to generate key pair.
Strange. I flashed it under GOS and it runs fine, but I'd like to be sure of my purchase while I can still send it back.