I've gone through 5 playthroughs and am in the middle of my 6th (mainly to just do dark mode and a couple quests that I missed). You can definitely get everything done and try all three skill paths and even some combination thereof in 5 playthroughs. Of course you can just be like some and reload the game from a certain point, but I prefer to start from scratch every time.
In terms of the 16 endings, if you're really hardcore about it, you could try to have all 16 possibilities. Otherwise, you can just craft the game in a way to try to get all of the different possibilities with each play-through, so that you can experience each once. Honestly, with the exception of a couple choices, the differences in the end aren't all too worth going out of the way for unless, as I said before, you're hardcore about it.
If you uninstall the game, it will keep your saves. Also if you want, you can just take the save files from your Documents folder (I think it's there) and put them to the side somewhere if you really want to).
So the answer is no, you don't need the game installed if you aren't going to touch it ever again but still want to keep the saves for a potential Witcher 3.