RE: Mixer Box Replacement
It sounds very poor, but this is the real problem with, especially, Fluence. 63 in the UK? The faults you get are simply beyond the experience of the guys doing the work. It'd make as much sense for you to do it yourself ... and that might not make any sense at all!
I wish you every luck that it will be remedied, but for me this is part of the heart ache of EVs at the moment. I value what they are, I love what they do, and it all seems to make so much sense. But at the moment the reliability is a big unknown and I need my cars to be there when I need them. I had a chance to experience this great tech when normally only well off early adopters would usually have them (and be prepared to tolerate a few early bugs). But I am no early adopter and so now I suspect I may have to wait until EVs become more 'mainstream' before I feel very confident about going ahead with my next EV.
all the best...