The PMIC and Fuel Gauge are left off intentionally to allow for different battery chemistries and devices that don’t need a charger (external power or disposable batteries). Also for applications like solar there are better PMICs that the bq24195 so it made sense to leave it off.
3rd-party SIM cards are not supported for the B Series SoM B402 (LTE). There’s a MFF2 SMD Particle SIM on the SoM. That being said, the SIM pins are brought out to the M.2 connector. The problem is that you need to make a modification to the SoM to disable the MFF2 SIM. This is possible, but not supported, and not practical at scale.