Lyman,
While playing armchair engineer is great fun, I am sure Yuasa has a lot of very bright boys and girls who do this for a living and have a level of experience and expertise in the subject you and I can never aspire to. I am quite confident that the cell design and testing is top notch - dare I say on this forum, from my personal experience even if in another area, more so than they would be from any US manufacturer - and that the cells will not cook off if operated within their specified envelope.
Likewise, designing electronic circuits to meet certain current and voltage targets is almost a trivial matter these days and I would not look for any design problems in either the balancing units or the charger - with a caveat on the charger in that it apparently implements a novel charging algorithm. I'm confident that the electronics faithfully applies that algorithm, but how good is the algorithm? "Novel" does not exactly square with "well known".
What it boils down to then is the system integration aspects. Despite the complexity of the individual items, it really is no different from ensuring that a bolt or rivet isn't stressed beyond its specified limits - either that or specify a better rivet. The key factor here is whether the aircraft system as a whole - the one great unknown before the aircraft started flying, since it too is "novel" - is respecting the specifications provided to the manufacturers of the various components.
Of course the investigative bodies have to take things step by step and rule out the more obvious culprits first. My reading of the reports to date is that this has been more or less done with the batteries and electronics. Leaving....?