[StrlngSlvr06]

Bear with me. I'm fairly mechanical and have done my own vehicle maintenance for the last 16 years and have decent tools. I have a K+D CAN cable, I have ISTA+, INPA, and various coding programs and have familiarized myself with them to gather live data, check ECUs for codes, diagnose, pull service directions, etc. I have had several Volvos up to MY'10 but BMWs are new to me and I am looking for insight from someone with experience. I bought a very clean and maintained 2008 E93 N51 with 110k miles in June this year from the original owner. They had all the service paperwork from the day they drove it off the lot and only had maintenance performed at the dealership. I've done lots of research, here are my symptoms, and my ideas for fixing the problem. Tell me what you think and why.

First some things I know about the car that are relevant:
1) N51 (SULEV) has secondary air pump to assist in complete fuel burn in the exhaust as well as the stage 3 intake with 2 DISA valves
2) The water pump and thermostat are original and have never had any maintenance
3) In November last year before I owned the vehicle, the car went to the dealership for a check engine light to change the 2 pre-cat o2 sensors
4) I have replaced the original post-cat o2 sensors as well as the original spark plugs (spark plugs far overdue - little if any electrode tip remaining)
5) I have replaced the air filter with an oiled K&N filter (unplugged and separated the MAF housing when replacing the air filter, but everything went back together fine)
5) I know that when looking at live data for the lambda (o2) sensors, the pre-cat voltage should fluctuate between rich and lean and the post-cat o2 sensors voltage should stay constant.
6) SULEV models have extended warranties in specific states, the car is registered in Colorado, which is not one of those state.

Symptoms:
1) MIL came on recently with P0420 and P0430 Catalytic Converter Below Threshold - I thought maybe it was the post-cat sensors so I replaced those (mentioned above) and used ISTA to clear the codes (the only codes that have appeared in 5 months of my ownership)
2) Codes returned shortly after on my second cold start since clearing (also appeared before the car had reached operating temperature)
3) Now that the weather is cold, the car takes a very long time to reach operating temperature - almost 20 minutes in 38 degree weather, interior heat isn't amazing but not intolerable.
4) At operating temperature, the bank 1 post-cat Sensor fluctuates voltage. Bank 2 post-cat sensor does not.
5) Exhaust has been extra sooty, with a white car its very apparent

So these are my thoughts: The car is running rich, causing sooty exhaust and throwing the cat codes. If the fuel isn't burnt completely (bad spark plugs? not enough air?) the unburnt exhaust fumes could melt the cat filaments. But if the thermostat is stuck open (long warm up times and bad interior heat) the fuel map will never get to the closed loop and continue to provide a rich fuel mixture that could potentially, destroy the cats.

Am I right to assume that I should try to replace the thermostat (and water pump as it is original, overdue, and I'm already working in the area) first?
Would the DISA valves in the intake manifold cause these symptoms? Am I overlooking the use of the Secondary Air System?

This is how I would diagnose this, but I'm trying to save time from unnecessary work:
Check DISA valves for operation?
Check for Intake Leaks behind the MAF? (although I think I would run lean, not rich)
Bad/dirty MAF Sensor?
Check for exhaust leaks letting air in or out before the post-cat sensor?
Check operation of secondary air pump and diaphragm?
Check thermostat? (Is there a way to test it?)
Check fuel trims for verification of rich mixture?
Check engine temp when codes where thrown?

I need help; where should I start? The most expensive option seems to immediately replace the cats. If it's running rich, they'll fail again.