Sorting by "Date Added" in a Folder seems to order by the Date the articles were added to my library. I would expect it to order papers by the date they were added to THAT folder
Yes, you can only sort by the "Date added" timestamp which is the date when it was added to your library.
We currently do not keep track of when references are moved to folders or labels. We also don't have any plans for this feature at the moment.