You don't connect the negatives to the chassis.
Perfect. This is much better than connecting the negatives to the chassis.
Based on many diagrams I have seen, including several from Victron, items like the inverter and the SCC that have separate case grounding terminals, wires should be run from those grounding terminals to the negative bus bar. Then you can run a wire from the negative bus bar to the chassis for a ground.
I have those same two batteries in series in my system. Once they are properly balanced before connecting in series, they will stay balanced for a long time. I have setup midpoint voltage monitoring in my system (using the VE BMV-712). Based on this, my two batteries are still at the same voltage after about 8 months of use. If the time comes I can always take a day to rebalance them.
Good video. It might be overkill but I would add two things. 1) If the two batteries have a large difference in initial SOC, I would charge them individually first. It's my understanding that hooking up two batteries with a big difference in SOC can cause a large current between the two batteries as they try to self balance. Once they are close you can connect them in parallel and continue with what is shown in the video. 2) After the parallel charge completes I would disconnect the charger but leave to the two batteries in parallel for at least a few hours. LiFePO4 voltage settles after a full charge. Keeping them in parallel can help ensure the settling is balanced.