Once again, CPOM is on the money. Low vacuum to the fuel regulator=higher fuel pressure, your computer won't know about the vacuum leak, only that it has switched on the vac switching valve to the regulator, lowering fuel pressure. The pulse width at the injectors is set for LOWER pressure WITH vac, so you're dumping fuel, contaminating cats and freaking the O2 sensor.
It's quite possible the first jammed cat burst the intake gasket with hot backpressure. Auto computers can merely tell what is out of the norm, not why or what to replace. It's up to a good tech to analyze codes and determine a significant order of importance. Cheap scanners will show codes, but not values sent by senders. Half a dozen codes pumped out of an exasperated ECM might only really be a vac leak or weak temp sender.
I too, think the harness is BS. Your truck is 10 years newer than mine and surely has much less miles. My wiring is in perfect condition save for a few layers of off-road enjoyment. Perhaps there has been some chafing on a wire or two but certainly easier to repair than replace the whole dang thing! Try moving the harness yourself.
Change the gaskets and post the results for all to know, just 'cause it hasn't happened to us yet doesn't mean it's not going to...Cal