Can you post the rest of the readings from the snap-on scanner. I use 1 all the time and am good at reading it.
With the replaced ecm and it read no data I bet that the replacement ecm is junk or the wrong 1 .
With the check engine light on the system will stay in open loop and will run rich all the time = very bad gas milage + fouled spark plugs =a good way to burn out the cat.
You said the ecm was not working because the check engine light was on well that is what it is suppose to do when it detects a problem.