Dealer can look it up. No a "normal" person cannot if it's not yours already. If it's yours and is in your own stellar support account you can look at it I believe. 

No there is no RTK activation for any JD display. "AutoTrac" is the one that does the steering and it works on any GPS signal level. Way back when there was a little cheaper option for "AutoTrac SF1" which limited steering to only on SF1 signal. Not super common and most were upgraded or have the newer "all signal" activation. AutoTrac and AutoTrac SF2 are the same thing. 

Challenge code is what's used when doing activations over the phone. Or deactivations, transfers, that sort of thing. Means nothing to a user really unless doing those things. It will change after doing any of those things so the challenge code is not permanent. 

Yes, those are some if not all of the activations available. Machine Sync might be another one? But not real common back then. Some make it more valuable like AutoTrac and Section control. Pivot pro, rowsense, ITEC pro, are all more niche things and not everyone needs those so they aren't as valuable. Basics is needed to do anything on the screen. It's not paid, it comes on a screen when it was new.