To get back on topic here for sec, I'm going to take a wild guess here and say that the P0421 code didn't come back after the cat was replaced. If it did, with the misfire codes in there it cannot be trusted. Seeing that the Misfires were cyls 1&3, I'd say just the wires were at fault, but should have the coil boots replaced. Sometimes they don't have boots available for some applications, then they have to replace the coil. To verify the repair, on the catalyst anyway, they should have driven an OBD2 drive cycle, which is a specific set of driving conditions to get the PCM to run all of it's tests, then check for pending codes, and look at mode $06 data to see if it was passing. It's not that tough, if the tech knows what they're doing, and how the system works....