MIT Mystery Hunt 2020/The Balloon Merchant's Gambit

The Balloon Merchant's Gambit is a variety puzzle from the round of the 2020 MIT Mystery Hunt.

Solve Path
The solver will need to reconstruct the chess board to get anywhere. Beforehand, the solver might note the names of the moves as rather interesting; dropping the last word gives a crossword clue. The solver might also notice that the first letter of the last word is both an enumeration for the clue and an indication of how many spaces the piece moved, perhaps noting the connection after solving a number of the clues.

There is one easily known move at the start, since only one kind of move moves two pieces at once: 6. ...O-O (SRI/SIR). To make way for castling, both the g8 knight and the f8 bishop must have moved. The only black knight move beforehand is on turn 2 (IDEA). This starts the bishop off on an R, which is the only piece that can make a move of length 6 and thus must be the piece moved on turn 5 (Bxa3, leaving RADIOS). This solves 5. Ra3 (YES), 6. Nxa3 (EELS), and 3. a4 (ESP). This in turn forces the other knight to be white's fourth move (PAWN).

Black's turn 7 (CUBA) must be the other knight, since white can't capture it by the next turn. White's turn 8 must be moving the kingside knight as well.

Next, we do some accounting on the clue in Black 10: both sides are even on material at that point. The first nine moves are known to have have no captures; then there is a rook-bishop trade in black's favor. Black 6 is castling, White 7 is a pawn push, and Black 7/White 8 are non-capturing knight moves (neither end in a letter that an enemy-side move ends in, and neither have made enough moves to reach the enemy home ranks). Thus, moves 9 and 10 on both sides are the only captures after that rook-bishop trade. White 9 must be the pawn moved in White 7 capturing the pawn moved in Black 3, and Black's counterplay is to recapture said pawn; as such, the 6-square move on White 10 must be the bishop. Since the letter positions of three minor black pieces are accounted for and the fourth is not reachable from the bishop's current position, this bishop must trade for a rook using 10. Bxa6 to make up the difference in material (forcing 8... Ra6 (ORO) and 4... a5 (ROW)). From there, the only possible capture is 10... bxa6 (SO), and the path-clearing move (DA) must be 9. cxd5 (forcing 3...d5 (ADA) and 7. c4 (QED)).

The S on b7 means the knight move on black 7 (CUBA) must be ...Nd7. It cannot reach d5, so AIDA must be covered by the other knight (meaning 2... Nf6 (IDEA)).

The bishop moves must now be enabled by 1... e6 (AI) 2. e3 (TT).

The only valid move for Black 12 (SOS) is 12...h5; for Black 13 (PAN), 13...f5. This locks in 4. Nh3 (PAWN), 8. Ng5 (NOSE), and 11...Nxe3 (ABUT). The only move capable of making APT at this point is 12. Bxe3, meaning white's first move was 1. d4 (PUB). White's 11th move was 11. Nc4 (SLED).

We have now reconstructed the board state to the start of move 13, and the next few moves are constrained heavily by the letters:

[I]

13. h4 (SNO) f5 (PAN) 14. g4 (AWE) Nb6 (ARSE) 15. Nxb6 (DINE) cxb6 (RE) 16. Qb1 (BAE) Bb7 (US) 17. f3 (DL) Bxf3 (SNAIL) 18. Rh2 (PS) Qd6 (BAD) 19. b4 (ELO) Qxh2 (DRAWS) 20. Kf1? (AS) fxg4?? (NE)

Having finally reconstructed the game, we determine the checkmating move: 21. Qh7#, crossing the answer word EQUINES.

Puzzle Elements
Flavortext - ''Life-sized chess pieces stand on a giant chessboard that's been painted on the ground, but some vandal has scrawled graffiti all over it! A balloon merchant standing by some of the pieces attracts your attention.''

Clue-Centric - Rather interestingly, the first words of every move constitute a crossword clue, which can be solved.

Alphanumeric - The last word in every move starts with a letter B-G...

Enumeration - ...which serve as enumerations to their corresponding crossword clue, in addition to indicating how far the piece moved.

Chess - The puzzle is (rather blatantly) themed around a game of chess.

Retrace Their Steps - The puzzle involves reconstructing the board state and the sequence of moves that led to said board state.

A Letter in Every Space - Each move crosses squares that spell out its corresponding crossword clue, in order. The final answer comes from the unclued checkmating move, 21. Qh7#.

Hint in Title - While the puzzle may be solved completely agnostic of this hint, the title, in conjunction with the Enumeration word, clues that the answer has seven letters.

Hint in Flavortext - Some vandal has scrawled graffiti all over [the chessboard]!