The drawing.
First much easier to read without the DC-.
What are the dashed red lines in the attached image?
I think you have made the switching too complicated. By my calculation there are 36 different combinations in the switching, (3 x 3 x2 x 2). If you switch to 1 DCP and 2 simple on/off switches were will only be 12 possible combinations and they are easier to understand. Each battery is either on or off and connected to their respective systems (start or house) or they are paralleled.
For engine start system, if the "Engine Master" switch is located near the start button the buzzer should not be much of an issue. If I recall correctly you intend to put the master switch below decks, if so this will require a trip below decks every time the engine is started or stopped. In an urgent situation when it is necessary to start the engine the extra time and effort could exacerbate the situation. Hiding a master switch below decks would be good for security, meaning hard to start the engine if it is being stolen. Placing a secondary switch, doesn't have to be keyed, near the start button, would eliminate the inconvenience and make the system safer. Thus, have one master somewhere out of sight and mind and a second easily accessible switch in series.