Say I have a copy of Llanowar Elves from the 2017 Core Set. Can I put it in a current Standard deck, or do I need to have a version from currently Standard-legal set?
Yes. Answered here.
Yes. The only thing that matters for determining card legality is the name of the card. As far as the Oracle is concerned, 2 different printings of shockare the exact same card for all game and tournament rule purposes