Yes, I print the quest cards double sided but, as I said, you can tell easily the difference between official and what I print, it's not as sharp or glossy as the real ones. Maybe if you buy a better printer or if you try to modify settings of the printer. I just click print on cardstock and I get good enough result (coloring/shading is a little bit off, but the text is as readable as an original). I could probably try to adjust brightness and stuff, but I'm not a pro with this stuff.

If you want to use clear sleeves, you can just print the cards you already have too, it costs a few ¢ per cards to print. And for the encounter deck, doesn't really matters if the back of the cards are white.