Is the sideboard simply adding 2 cards into the deck? Or subbing in up to 2 (potentially) useless cards for up to 2 (potentially) useful cards?
The former. You are just adding in the 2 extra cards, you may not remove any cards from your original.
We keep any sideboarded cards for the remainder of a match, correct?
No, the deck is reset to your original deck after each game.
We can only use cards from decks we've already played?
Cards can come from any of the other decks you have made for this round. This is to not force schedule issues and teams trying to schedule their matches in a specific order.
After each game, is our deck reset to original? Like, G1 loss, we sideboard. G2 win. in G3 do we use original deck or the 2 sideboard deck. If we lose G2, then can we have further 2 sideboard cards?
Reset to original correct. If you lose G1, then in G2 you may sideboard. If you win G2, then G3 goes back to your original. If you lose G2, then G3 you may add sideboard into your original deck.