That is an AC source selector switch. Used for shorepower/ generator switching or possibly shorepower/inverter switching. Shown below.
The 2 shunts are for battery monitors (one is visible in the upper right) and/or ammeters.
Your plan sounds good to me. Yes, multiple connections are allowed on the busses, up to 4 per stud although I try to keep it to less when possible.
What size cable are you running to the fuse blocks?
Living aboard in Victoria Harbour