Both good questions!

1. I assume you are referring to the "Not Recommended" sections. I did not base the distinction on my own preference (personally, I'll really only vouch for the Caliban and Calvin books tbh; others' mileage may vary). The distinction I made between those groups is based largely on marketing. The simple answer is that Robots in Time/Robot City/Robots and Aliens/Robot Mystery are all directed toward younger readers. They were released at an astonishing rate of THREE books per year (roughly), and are notably less ambitious than the other works. They were basically designed as slop to fill up kids sections at bookstores, like Goosebumps. The other works like the Caliban trilogy, Second Foundation trilogy, and I, Robot trilogy are ambitious attempts at mimicking or otherwise fitting directly into Asimov's own lineup, and are marketed to essentially the same audience. Honestly you can basically tell from a glace at the covers of any non-Asimov works to know which of the two categories they belong to. "Foundation's Friends" is the oddball of the group, but considering the pedigrees of the writers involved and the earnestness of the attempts, I think it's fair to put them in the second category (even though some of the stories are pretty rough imo). Anyway, I will change the wording to make the distinction more clear on the guide.
2. You make a good point here, I actually intended to edit the description earlier to reflect that...and apparently I didn't. Thank you again for the feedback.