The temperature readings make NO sense. The catalyst is colder than the inlet, so either the catalyst is dead, or the thermometer is inaccurate. Neither is a surprise to me, I've been saying the catalyst is likely to be failed since Post #4. The muffler outlet is the hottest part of the whole system, and that's crazy.isn't the temperature of the cat supposed to be higher than the rest of the system if it's working properly? I'm starting to think that may be at least part of the problems.
I'm guessing half the problem is failed catalyst, half the problem is due to differences in emissivity of the exhaust system.
Nobody wants to talk about the inaccuracy of infra-red thermometers because of emissivity. The thermometer reads infra-red, different surface finishes and different materials emit infra-red at lower or higher levels even at the same temperature. Not many consumer-level infra-red thermometers even have an adjustment for emissivity; and if you have one that's adjustable, you're still just guessing at the emissivity of the object you're measuring--although we know that shiny stuff "reads" colder than it probably is.
https://en.wikipedia.org/wiki/Emissivity