I am not an electrical engineer. I am not any kind of engineer. Try this at your own risk.
My 05 Met II also suffered from the dreaded "siesta syndrome". Perfectly healthy, just did not want to work.
I followed the schematics in the Honda shop manual and came to the conclusion the main relay was not getting energized. That is the relay tucked in behind the ECM . I looked high and low for the reason, fuse, wire, bad battery bus connector. Thought it might be a diode in the main relay that was not holding the relay closed. Relay tested good. Could not locate the problem. Finally decided to route around the whole circuit and power the main relay from the battery positive pole. I ran a jumper from the pos pole on the battery to the red/green connector wire on the main relay. Viola. Power everywhere. Lights, horn, starter. Life is good. That is how I run it now. It bypasses the ignition key circuit. All I have to do is hook the alligator clip to a crimp connector that is attached to the pos battery pole.
I spent hours trying to trouble shoot this problem. Had very knowledgeable other people take a look. There is basically no way to trouble shoot the ECMs individual internal circuits as with a code reader. You trouble shoot the component at the other end of the wiring, the wiring itself, and if they are good then Honda assumes it is the ECM. I may have a problem with an internal short in the ECM, I don't know. I don't care. I spent hours going thru every wire in the main harness, the start cirucit, the ignition key circuit, replaced the main relay because it was the cheapest component. Still did not solve the problem. My reroute did solve the problem. Works wonderfully. Make sure you have a good secure connection since the jumper runs directly off the battery the potential current draw can be high. With a poor connection you will either burn up the wire or the alligator clip will get so hot as to cause a very bad burn on your fingers when you touch it to disconnect. You need to disconnect when you park the scooter because the whole electrical system is powered continuously. But at least the sucka runs and I ride.