I'm rewiring my engine control panel on my new-to-me boat after a few alarms weren't working, then the alternator was only kicking in when I'd rev the engine, then finally a dead short that melted some wiring. In short, a mess.

After looking at the circuit diagram for the 400th time I'm starting to understand it. But I have a few questions.

The diagram has a wire coming from the alarm negative to post 1 on the circuit board, then it goes to pin 2 on the harness plug. Then pin 2 on the other end of the harness plug has nothing connected- it shows just a + symbol.

The circuit board has a couple of components on its backside. Perhaps the alarm negative is fed through them. If that's that case, why would the diagram show a wire going from post 1 on the circuit board to a blank pin 2 on the harness?

Yep, Good luck. I rewired my Bukh engine panel, and got rid of the circuit board and key switch completely... put in push button start/stop
The wire that goes nowhere is OK from memory, basically the Piezo alarm uses the potential Voltage difference to alarm...doesn't need to connect to anything. The resistor is there as a load to kick the excitation in on the alternator, and yes you need this, and still need to bring the revs up a little to kick the alternator in.
The circuit can be simplified, see below. From memory I had to change the direction/position of diode as alarm was going off...forgot to update drawing...sorry