There’s a “Collection” button that displays the items you’ve discovered in your sessions, though it doesn’t give any extra info beyond what you already see in-game.
Chords are when you left-click on a number that has the correct number of adjacent flagged cells, automatically revealing the remaining surrounding cells.
Bonuses are, as you said, "Efficiency" for finishing under a certain number of clicks, "No flags", "Perfect game" (no damage) and "Fast clear" for finishing under a certain time.
Hopefully everything will be clearer in the stable release :)