Just discovering the 'Tray' Folder :(

From previous (unfortunate) experience, I've deleted tray files to save space however on reloading the game noticed that all of my saves have gone and I have been devastated. As you mentioned in the original post you are aware of this, but as far as I can figure out there is no definitive way to judge which of the tray files correlates to your library files. You could try deleting some sims families you don't want anymore individually and then checking if any files are missing from the folder but writing down the file names? Normally, I delete the screenshot folder only and then delete various things in-game so I know exactly what will be going! In my honest opinion, I wouldn't delete the tray files unless you want to risk losing your entire save file. There may be others who are a lot more savvy with this than me, of course. I just didn't want you to lose however many hours you've put into your game!