For item #1, add column Date Added to your Collection display and sort descending by date. Your recently added tracks will be at the top of the list.
Item #2, the Collection is "All Tracks" in Rekordbox. If you delete it, you are deleting everything. There should never be a reason to delete the entire collection. I run RB on Windows 10. All my tracks are located in one folder that I import from, 16K tracks. Importing the same folder only brings in new tracks added to the folder. I suggest that you have one primary folder and if you chose to create sub folders for organization of tracks. You can then add new tracks to these folders and import the top folder to get only new tracks.
In Windows Explorer, there is a Date Modified column that acts in the same manner.
