It's not much of a surprise it wouldn't start when there was no spark at the spark plug. It sounds like you may be moving towards a solution with your recent work cleaning the ECU connections. When I check for sparks I use a spare spark plug, connect it to where the actual spark plug normally goes, and hold the spark plug threads against an engine ground and look for a spark at the tip when a helper cranks the engine. The spark is very easy to see, quite bright, bright enough to see in daylight, but it is small in dimension. So it is hard to say whether the spark you are seeing is correct or not. One idea, compare the spark you see on your car to a similar car that does start.
If you've got a good spark but it won't start, be aware that your prior starting attempts may have flooded the engine. Gas gets injected whenever the engine is cranking, and over time you can get quite a bit of gas in the cylinders when there's no spark happening to burn it off. Remove a spark plug. If the tip appears wet, remove them all, disable the spark system, and crank the engine a few times to expel as much gas from the cylinders as possible, then let it sit a day or two for the rest of the gas to evaporate.
Let us know what you've found at that point.