If the SoC is going down while you're constantly spoofing the BCM, that means the pack voltage is too low. I've seen this in my testing when the pack voltage drops below 115 volts. The MCM is using the voltage data it receives from MCM'E' connector to override the data the BCM is sending over BATTSCI.
Connecting separate grid chargers to each cell/module/etc is going to unbalance the pack... unless you have a BMS that can rebalance afterwards. There's really no reason to charge cells/modules/etc separately. Just get a charger that can handle the maximum charging voltage and you're all set. Without a BMS, you'll need to manually monitor each cell with a voltage meter.