“it may take a while for it to work out the battery was fully charged and potentially cause gassing for a while as it tried to further charge it itself. But who knows?!” I can asure you this would never happen. The alternator regulates the output to around 14.2 Volts max so it will never cause gassing unless there’s a fault in the regulator. Even if you drove the car every day for several hours the battery would never reach 100% SOC due to the capped charge voltage of 14.2 Volts. It might charge to about 90-95% maximum SOC via the alternator. The CTEK or other good chargers will raise the charging voltage to 14.4-14.7V depending on ambient temperature and this is the only way to reach 100% SOC. You can check this yourself after charging and starting with a multimeter or using an ODB scan tool, It will never be higher than around 14.2 Volts with engine running. Gassing the battery is actualy a much higher risk using an external charger(especially a poorly bad designed one!) than via a modern car’s alternator.
Regards the BMS, The CTEK might confuse it as it also uses ‘PULSE’ type charging during maintenance mode. Pulse charging in nothing like the normal steady current charging the battery expects during bulk, absortion charge stages or during normal driving. But we also don’t know if the current sensor can sense actual direction of current flow, so it could also be confusing the CTEK charge as a major parasitic drain. Unless we can get detailed specs on the current sensor and the BMS algorithm we will never know for sure.