If your "loaner" ecm was running rich when the damage happened that was not the problem. Rich doesn't kill things.
Lean does. Rich also cools the combustion temp a little especially the tops of the pistons... Fuel is a bit of a coolant before it ignites. The more you have the cooler things tend to be up to a point.
The loaner ecm did not break your piston.
You never said where your work was done and who did it. That kit is not something for the novice imo. However, I'm not calling you a novice either.
If you had two engines with issues, which one did you put together? How far apart were they?