OK, that is all as expected and as you say doesn’t move things forward. There is normally no distinction between “common; and “negative” in a conventional ” negative earth system”.
The strange mix of issues which you describe is almost certainly caused by a “ground rail / earth” fault. Classically in this situation energising one circuit will partially ( depending on their relative loads) energise another. The failure of Dufour to indicate the earthing layout unfortunately makes this impossible to diagnose remotely.
Good luck.
The membership association for owners of Dufour Yachts