I'll leave the very specific questions to my colleagues, but to your question about 3rd party chips on the PCB:

Even though we have a closed source secure element and now a bluetooth chip, we do not trust these chips. The secure element does not store the private keys, it only stores one of the three secrets to decrypt the seed stored on the MCU. Of course the bluetooth chip never gets access to private key information and is only used for communication purposes.

/Joko