Also, swapping the modem chip would (well, should) be a permitted use, as the IMEI is meant to be bound to the modem. When a separate modem module is used, the modem is the communication device, not the phone.

The pinephone is more similar to a laptop with an optional LTE module rather than an integrated setup like an smartphone where the CPU contains the baseband. It's not illegal to swap out modem cards in a laptop anywhere AFAIK.