If the engine is running, the ECU is definitely not 100% bad. If you have probed the injector and ignition outputs from the ECU and they all appear to be switching when the engine is running (no dead cylinders) then the probability weights on the side that the ECU is OK.
The ignition testing service loop is low voltage (12v) between the negative terminal of the #1 coil and the switching transistor in the ignitor. If you look at some inductive timing lights, specifically those meant to work by clamping the inductive pick up over the #1 high voltage lead between the distributor and spark plug, they will have an arrow on the pick up indicating that it should be pointed at the spark plug. The triggering circuit in some timing lights is sensitive to the orientation of how the pick up is aligned. If incorrectly aligned some will not work at all, some give flakey results and some are insensitive to the orientation of the pick up. It is a good idea to check your timing light on a running engine to confirm that it is not defective. If it tests out OK, try flipping the orientation of the pick up back and forth to see if that resolves the problem. Also, the loop is not huge and if you have a large pick up the jaws of the pick up may not be closing completely. If the jaws of the pick up are not closing completely around the service loop that will make for a weak trigger signal which may not fire the timing light.
Since you were messing with the ignitor, check the grounding connector G103 which connects to the 8 pin connector on the ignitor. Check the actual grounding lug (G103) and check the continuity of the pins in the C123 8 pin connector which connect to the lug. In fact, check all the terminals on the ignitor for signs of corrosion. 10 years ago it was not unheard of for people to have ignition problems because of flakey electrical problems with the ignitor connections / grounding. This problem seemed to be more common on owners who obsessed about having a clean engine.
There is a condenser on the ignition coil +12 supply for radio noise suppression. If the condenser becomes iffy, it can cause problems for the ignition system. Unfortunately, testing the condenser requires a rather uncommon tool. The connector for the condenser gives you access to the 12v supply to the coils so you can probe at that connection to confirm that the coils have a healthy 12v supply.
I will note that none of this really jives with your circumstances where the engine was running fine and then wasn't running fine. However, you now seem to be in the process of eliminating all possible causes so this would be part of the process of ruling out an ignition problem.
If you can get the ignition timing light working, you can do the basic test in the service manual which is checking the timing at idle which should be 15+/- 2 deg. The service manual gives no guidance for checking the timing at any other engine operating condition.