I had a Titan Protune with an MST intake, PowerNeedy Catted DP and ran map 4 without any issues. The only difference was the intake and coating between OP and myself. Hence it was a suggestion.
Welcome to catless life. If you're not chasing absolute power, a catless downpipe IMO is not worth it (did it once in a younger life). A sports cat should be good up to 650 HP.
Depends on the cat. Most built with a GESi or HJS Euro VI cat should not give you a CEL with a stock tuned car. That being said, there are a lot more other variables that can cause a car with a sports cat to throw a code (e.g. outside temperature, humidity, etc.)
I haven't had any issues with...
Clear CEL and drive it around for a few cycles to clear the permanent code as angrytoddler stated. If you need some help, I am in NoVA and can lend a hand.
I'm sure getting LTA approval for an aftermarket cat is going to be near impossible but could you get someone to fabricate you a catless downpipe in the exact shape as an OEM cat? I'm sure that would be cheaper than buying a cat and gutting it.
I have a PowerNeedy Catted DP myself (uses the same GESI cat). No CELs on my end. I did get it Swaintech coated prior to the install so that may helped with the P0420 code.
I would suggest giving Active Autowerkes a call and see what they can help you out with.
I ran my MST intake with the factory chargepipe without any issues. I also have a 2020 and not a 2021. It sounds like there are slight dimension changes between the model years.