I also experienced such a sudden loss of 8 or 9 exp several times - in all of these cases a very young general was involved. I just assumed that this is a bug (9 chevrons not due to several positive traits but just one negative trait which shifts experience below zero).
I think that command stars are not at all related to bodyguard valour (I am not sure, though).
As for the "unit experience", it is in fact an average experience of individual soldiers in a particular unit.
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