I’m guessing the thoughts ought be related to the thread topic, right? 
What I can say is that I am profoundly sympathetic (in fact, empathetic) to the general point of discussion here about the challenge of orienting the texts especially if someone is new to them. I would desperately love to see the coming to be of a “noob-friendly grounding package”.
I’m sorry but just at the minute I’ve only had the chance to skim, and it’s already probably been covered, but yes, Snp1.8 and Kp9 are the same text: Metta Sutta translation challenge!.
I had also understood it so that the kp was a kind of handbook for novice monastics. The Khuddakapāṭha Introduction on the Dhammatalks website also points in this direction.
With respect to why the SC kp9 and Snp1.8 lists aren’t identical, in short: I don’t know. I can look to see if there is any technical reason.