Tony, I am probably one of the few people with direct experience of what you are doing as I have a very similar system, a hybrid LA/Li set up with B2B's, long wires controlled by a BMV 713 and a BEP switch. I do think you are overcomplicating it a tad! There is two sides to this i) the technical set up and ii) how it is operated in practice. Lets look at these separately.
Firstly the technical bit. For me the first level of control has to be the charge source reducing its input when a set level is reached, rather than the isolation switch activated. You say your first line of defence is the isolation activated when you get to 80% SoC. If you are using solar, then the MPPT will work fine to turn down to float so your isolation switch will never be needed. If you are on the alternator for charge then the B2B's will turn down once the set level is needed. I assume your isolation switch is the BEP. In the 4 years I have operated my system, my BEP has not activated once (on its own). I test it infrequently. For me the first level must not rely on the isolation switch.
You then say you need 3 BMVs. Why are you controlling on SoC? On my BMV, SoC is pretty useless after a couple of weeks since sync. Why not use voltage? My BMV is set to trigger the relay for high voltage at 13.9V. This is an absolute value. My state of charge limit is always the same – when the alternator gets to 13.9v at circa 50A (drops from 120A at start). The SoC reading could be anything. One advantage of using voltage is that the BMV has a high level and low level relay so the one BMV does both. That removes your 2nd BMV, and also allows your charge sources to shut down rather than relying on the BEP.
The BMV voltage is my second line of defence but as above it is never normally triggered. My third defence is the BMV sounding an audible (just) alarm at 14.0V volts, then a fourth - a cell monitoring device that activates the BEP if an individual cell goes above 3.8V and then a final audible alarm if the whole lot goes over 14.4V. The automated isolation is all via the BEP switch which is a vulnerability in the way Tom's Tyco bipolar relay could fail, but shutting down the charge sources in normal operation has to be the way to do it.
I think though that you have to look at normal operation. I very rarely get anywhere near isolation events. I see 2 modes of operation, moving and stationary. An example from this summers 3 months trip out. In the stationary mode, we are tied up not going anywhere. Our 200Ah of power has gone out overnight (and previous 24hrs) and the BMV is showing -200Ahrs. The sun starts charging, we run the engine for an hour to warm the water (putting circa 100Ahr back in). The sun does the rest and the mppt goes into float at the end of the afternoon. No intervention at all. No B2B running – just straight from the solar to Li's. Lack of sun one day and I run the engine for an extra 30 mins. Never any chance of getting near full charge with the engine. I will turn it off after the time I estimate. If I died in the interim, the BEP would isolate it. In moving mode, we are down -200Ahrs at the start of the day. I know we will be charged enough after circa 60 mins and the B2B will shut down the charge then. In practice though I have given up with the B2B's (2* 60A) as they consume so much power and heat up the boat! …..and I just manually isolate the Li's when the voltage gets to circa 13.8V. If I want it automatic then the B2Bs are fine but I am always watching the instruments as we idle along so spending 10 secs to turn a switch is not a chore. It really is so simple.
Now the solar is loosing its efficiency, when stationary, we just run the engine for 90 mins instead of 60 – but we are a very heavy power user.
I do use 2 BMVs, one on the Li system that does all the controlling and one on the LA system. Both are needed to understand battery health for the two banks.
Anyway, well done on setting up your system. It sounds complicated to the non initiated but it isnt. I would certainly encourage you to move to isolating the charge sources at source and then using voltage to control the BMV rather than SoC in an emergency situation.
How do you monitor your system with the 3 BMVs? I have mine wired into a Raspberry Pi which dumps all the data to the Victron server (data 24/7) so can see all of the data from the last year from my armchair.
For peeps new on here who dont know me, (who can you trust on the internet these days?)my electrical knowledge is significantly lacking when compared to Nick and Simon (MP) so my system was developed from a 'user' approach rather than a technical approach and one where I could marry 2nd hand Li's to a new boat where I didnt want to void any warranties – hence the hybrid. I do however consider myself an expert on Li batteries, particularly in their safety as I am director of a company working closely with the aviation industry setting standards (via destruction testing) on their transport on passenger aeroplanes and also developing new technology for early warning of battery failure leading to ignition or explosions (ie during dendrite failure).