Short and open each component in turn and then all combinations of any two components.
So 2 conditions for individual components, and 4 combinations for each pair of any two components.
eg D21 short + D22 short.
Why would that happen?
Who knows? - But, working out why it DID happen after lithobraking is harder.
So have D21a + D21b, D22a + D22b.
Likely needed? Of course not.
Silly? Maybe not.
Over-voltage and under-voltage the converters.
Low-OK, High-OK, OK-Low, High-Low, Low-Low, High-High.
The last two may only be transient to not be fatal.
The pin 5 commonality looks unwise and unnecessary.
Joining the two sections in any way if not essential should be avoided.
I have had an isolated sense IC incinerate itself and thus breach the isolation barrier - not allowing any possible path and as few impossible ones as you can is wise.
Vin clamping of converters is fatal even with two batteries BECAUSE OF battery ORING - a S/C converter takes down 2 x batteries. Better a dud battery taking down 1 converter than a dud converter taking down 2 batteries.
A polyswitch resettable thermal fuse in each Vin may help - or non OR'd batteries - or a hard fault trip on high Iin for more than a set period.
Decide what protection circuits you need in your batteries. Do they need per cell high and low voltage and overcurrent and ... ?
These are unlikely failure points but not impossible ones.
You MAY be better off with 3 unprotected cells per back and then per pack protection circuitry tailored to what is liable to perhaps go wrong and perhaps avoiding what is unlikely to happen, if protecting against it may result in higher overall risk.
Cooling at even quite moderate altitudes 'gets worse" and at real-sounding-rocket altitudes converter cooling may need radical rethinking.
Note that water is a marvellous thermal sink - 850 litre-degrees_C per kWh.
What would Murphy do to break this with minimum faults?
What WILL Murphy do to break this with minimum faults?
The big boys do it too:
Gemini 6A, October 25th 1965 (were you alive then? :-) ) failed to launch when an anomaly was detected 1.5 seconds AFTER engine ignition - but before clamps were released at 3.2 seconds, because "a plug fell out" in a connecting umbilical and created an unknown circuit condition.
[THere was a second as then unknown fault that would probably have terminated launch at 2.2 seconds as someone had left a sealing cap in a gas generator and one engine would not have risen to launch power, probably !!!]