The "remote functions" are, as you say, accessible via BMW's API, but that's implemented in the TCB, not the ZGM. I don't ~think~ that API will be accessible via the OBD port - not least because it would seem to be too much of a security risk if it were.
I would hope that these days there is no possibility of using the OBD port to sniff/analyse/inject data on the car's internal networks. These days all vehicles have CAN gateways (Ok, on an i3 there are more network types than just CAN so it's called a ZGM and not a CAN gateway, but the principle still applies) and one of the functions of that gateway is to ensure that internal traffic stays internal (i.e. it doesn't get broadcast onto the OBD port) - rather like your ISP's hub/router that keeps your home network traffic from leaking onto the internet, and stops external devices from pretending to be on the insid
I would think a WIFI radio (e.g., access point) would be better to replace the 3G cell radio, or an upgrade to the 4G cell radios that is in the later models.
The upgrade may be the easiest to program in because there should already be code for it, but you are back at needing to activate and pay for cell service with BMW.