How in the heck did you manage to get the coil leads swapped? Yes, that will send raw fuel into the cat and cause it to overheat! It's a good thing you didn't have to call the fire department.

As for ECU errors, unplugging the battery for 15 minutes should clear everything.

You will want to recheck the plug connectors for the O2 sensors, clean them and make sure the male leads didn't get pushed back.

Gasoline really doesn't get 'old'. What you get is water and sludge, the latter plugging up the injectors. Remember, the bad stuff is on the bottom of the tank and gets picked up first.

How did you test the injectors? Pull the rail, put a cup under each one, and then turned on the starter? A circuit error is just the electrics, I hope you didn't 'stretch' some wires where the loom enters the spark plug cover.

If you have a running engine, have a helper follow you around the block. Keep the gears in 1st or 2nd and the rpms around 3K and see if that helps clean out the combustion chambers. Have the helper check for smoke out of the exhaust.
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Keeping it running is better than buying new