It won't be returned to the battlefield
If, in your first game against a meta deck, your opponent will draw dozen of cards, attach multiple energies and get a 200+ damage KO all in the first turn, your whole image of what the game is about will shatter.
Ulvenwald Mysteries can be part of a dedicated Clue plan, giving us a free 1/1 token for every Clue cracked, in addition to providing us with a plethora of Clues over time (since many of our creatures are going to give us Clues unless they die, and if they die, then the Ulvenwald Mysteries will give us Clues as to what happened)
Be forewarned that older codes may be for a sealed event, so if MTGA is not running that format anymore, you may not get anything
Theres no noticeable difference once theyre in the sleeves
I'll leave that up to you, just remember fingerprints contain oil and can damage some materials