Bonus sheet cards are not added to the Standard card pool. So unless there is already a legal printing in another Standard legal set, the card is not legal in Standard. As is the case with Fynn the Fangearer. You cannot include it in Standard decks.

And why would you not get Syncopate anymore? Until you have 4 copies of a card scattered over all its printings, you get another copy for your collection. If you don't have a printing yet, even if you have a full four copies, you can get that printing from the pack's set, unlocking a full set for that printing all at once. But getting one copy of that printing is nessessary for that.

Excess commons and uncommons contribute to the vault, by the way. Commons and uncommons also do not have duplicate protection when opening packs. Only rares and mythics do.