There’s a lot going on here. 7 mana over 2 turns to draw 2 and make a 4/4 flyer might be solid. The issue is it doesn’t let you use mana of any color to cast the spells you exile, so you have to generate treasures by attacking or be playing the mirror to use them (or hit spell + a land to cast it). This will feel real bad when you cast the adventure on turn 3, hit two spells you can’t cast, and they answer it before it attacks.

That said, card advantage, a solid body and ramp can be good. I think it’s likely too weak to see play in standard main decks, but could be a strong play out of the board.

Typically, unless you have a card that says you may do something with the cards exiled, you can’t “un-exile” them. However, this card says that you may play them from exile, meaning you may play a land or cast a spell from exile. You can’t use certain effects like Channel or Cycling because those specifically require being discarded from your hand.

A treasure token is an artifact token with the activated ability of “Tap, Sacrifice this treasure: Add 1 mana of any color”. So, it’s essentially a portable mana that you can use at any time (as long as it’s not tapped down by something), or an artifact token for decks that have synergies with those.