1. Yes, vehicles with 2 cats and 4 o2 sensors would use the other code P0430. Like the later model 3.6 impala's or something like a silverado truck with 2/3 cats.
2. The job of the upstream narrow band o2 sensor is to bounce lean and rich of stoich, where stoich is 450mv give or take. It's taking real time readings and doing on the fly fueling adjustments based on what it see's in the exhaust gases. The up and down motion of the data logs should be pretty constant, like what you are seeing. That .200-.800mv with some random .100 and .900's depending on throttle input and fueling needs of the engine. The more throttle you give the faster the changes go and range they go increases if that makes senses. You kind of already descibed that, the gap between the switches increases because more fuel and oxygen are inside engine and exhaust gases. Als if like DFCO (decel fuel cut off) was active the o2 mv's tend to dive bomb to .1 and when you are heavy into the throttle they will stay above .800-.900mv.
The rear o2 sensors tend to switch much less as the exhaust warms up and will level out like that if the cat converter is doing it's job correctly. Typical readings tend to be in that .550 to .700 range and pretty much stay there all the time. If the rear sensor starts to switch like the front sensor and the computer see's it more than a few times in a given amount of drive cycles it will trip the code.
As for the cold start mv readings are .450, it's mainly because the vehicles computer does not use the o2 sensors at all during cold starts. The computer does not care what the o2 sensors read under a certain coolant temp. There are tables inside the calibration that control what the fueling will be while the engine temp is colder than say 120-130 degrees. Typically a car will run around 12-13 AFR while it's warming up and switch to regular stoich ratio of 14.7 for regular gas once it reaches closed loop fueling at that 120-130ยบ coolant temp.
3. Cat life is always a toss up, it's a mass produced part that they say should last well past the 100k mile warranty but you never know. Pre-mature failure happens and actual engine issues can shorten the life of the part. Bad fuels, poor maintence, tons of short trips that don't allow things to get fully heated inside the cat can shorten the life. Carbon build up from not beating on the engine a little could do it too. The 3800 isn't the a super clean engine and prehaps part of the reason they did away with it.
As far as cats and o2 sensors go. Buy decent stuff at least. Several people have bought the Walker Direct Replacement cats with zero issues, myself included there. Easy to bolt it and been trouble free for me. O2 sensors you don't want to cheap out on. I always buy OE parts, no off brand parts or bosch parts on a 3800. Only AC delco or Denso sensors, either work great. If your's have over 150k miles, you should replace them for sure. They have made it way further than they should. Typical replacement tends to be around 100-125k in most cases.
This may not have answered every question but mostly all of them.
