I can't answer them all for you, I haven't used Samsung devices since Note 3 (mostly because they don't offer Snapdragon variants in EU and Exynos is crap). XDA-Developers (along with some searching on the Internet) might be more helpful for you.
From what I remember:
- PDA is the build number, CSC is the country-specific code.
- TMB is T-Mobile, yes.
- No, you don't need to stay with T-Mobile firmware. I am not sure if and how it affects warranty though, if that's something you care about.
- Yes, you will get updates, but for the region/carrier that you choose. Not fully sure about upgrade paths - if you are gonna do a clean install, most likely you don't.
- Generally, the newer the better. Though in the old days there used to be some differences - sometimes newer firmware did not include all features from the older one. Searching probably gives you more information.
