On my opnsense router

They should not care as long as it has a compatible SIM/modem combo.

Yes, they mount as ttys, e.g. /dev/ttyUSB0. In Unix, everything is a file.

It is better, but it depends on the audio for the difference. Also, it would probably be hard to hear the difference playing over a phones speakers. The weakest link in the chain is always the problem you notice the most. Having a good setup for amp/speakers and you can hear the difference. Using Bluetooth earbuds to mow the lawn, it doesn’t matter. Sitting in my living room on my nice stereo, I notice.