I think you’ll need a (red) connection from the positive side of the battery to the B connector on the key switch; the key connects this to C (for choke), S (for start) and A (for accessories) as needed.

Also: the black connection from the starter (ground) to one of the M connectors on the switch will permanently kill the lower cylinder?

In some systems the black/ground connection on the solenoid goes to a safety interlock mercury switch on the engine to keep you from starting the engine at high throttle and/or in gear??

Yes, I believe the M terminals should each connect only to one of the points; Since one of the points is always closed (grounded), shorting them together grounds both, killing both cylinders.

No, I don’t think so, the key switch is on the “hot” side only; If you want to power anything from the dash (bow light) you’ll need to bring a ground up from the back for that…. I put a fuse in the red wire (after the power take-off on the solenoid) to protect against any shorts in the wiring going up to the dash (or coming back, ie; for the anchor light)

The fuse is there to protect the wiring from burning up, and looking at https://www.westmarine.com/west-advisor/Marine-Wire-Size-And-Ampacity.html I see that the max amperage by gauge (for 12V systems and Marine wire) is:

10 gauge: 60A
12 gauge: 45A
14 gauge: 35A

The OTHER consideration is that thinner wires have more resistance, and cause more voltage drop per foot of wire per amp of current carried, and critical wiring is supposed to keep voltage drop less than 3% (0.36V) for a full run (both to and from the battery).

Here’s a chart (for 12V, AWG– NOT automotive SAE wire): https://www.westmarine.com/west-advisor/Marine-Wire-Size-And-Ampacity.html

Free web calculators:
https://baymarinesupply.com/calculator
https://boathowto.com/electrics/wire-size-calculators/

I calculated amps based on the resistance I measured on my start and choke solenoids, using ohm’s law:
AMPS = VOLTS (12) divided by OHMS resistance

For my choke solenoid I calculated 5.45A (must have been about 2.2 ohms), and 2A for the start solenoid (must have been about 6 ohms). Your resistance may vary!