Dan:
Measuring the resistance across the trigger coils without unplugging them won't really tell you anything as you can't know what circuitry is inside the ignition boxes that you're also measuring that way. Even though it's a pain, it will be more meaningful if you unplug the connectors on the ignition boxes and re-measure the trigger coils to compare with the tacho coil.
Having said that a multi-mega ohm resistance across the tacho pickup sounds high. I seem to recall som Rotax troubleshooting tips somewhere, either from the manuals or from Rotax Owner. Maybe have a look at those places for help also.
Another thing to try is to connect your ohm meter across the wires of the tacho pickup coil, at the gauge end and then start wiggling the wires between the pickup and the tacho. If there is a broken wire somewhere, this may show it up. For this kind of test, an analog meter is best as the digital ones jump around in the readings mostly and it can sometimes confuse the readings. On an analog meter you will see large movements of the pointer if you have a broken wire.
I'm not quite sure what you were measuring when you were measuring across the gang connector on the firewall, so I can't comment on that.
The DC measurements from the tacho pickup are meaningless. The AC measurements sound a bit low to me, but explain why you didn't see any tacho reading until a certain speed with the diode in the circuit. A 1N4004 diode is a silicon diode that requires about 0.7 volts before it conducts. When you had the diode one way, it completely blocked the signal in the polarity the tacho gauge needs, in the other polarity it blocked the signal until the signal reached the forward bias level required to make the diode conduct. This experiment only confirms the AC voltage readings you took.
Given that the resistor across the tacho leads did not work, my best guess would either be a faulty pickup coil, a broken wire in the path from coil to gauge, or a faulty gauge. My strongest suspicion would be a broken wire between the pickup coil and the firewall. This is where the maximum flexing occurs in the wires due to the engine moving on its mounts.
HTH