Solution: The Quarrel at Teatime at the Museum

The clues

• 
  WalterThe estate’s groundskeeper.

  He was beside a crate.

• 
  SofiaThe chess prodigy.

  She was in the fifth column from the left. She was in the sixth row.

• 
  TheoThe paperboy.

  He was in the Workshop.

• 
  LenaThe painter who never finishes a canvas.

  She was in the fourth row.

• 
  MateoThe waiter who hears everything.

  He was beside a statue.

• 
  Vera · VictimThe widow who sets the crosswords.

  The victim. She was alone with the killer.

• 
  NoraThe village doctor.

  She was on a rug.

Step by step

The order our solver finds. Each step uses only the clues and what the steps before it proved, and its heading links to the technique.

1. Last cell left Every other cell is ruled out for Sofia, by her testimony or by a row or column already taken. Placed: Sofia: row 6, column 5 (Café).
2. Pinned suspects Lena can only be in the fourth row, so nobody else can be there.
3. Pinned suspects Mateo can only be in the bottom row, so nobody else can be there. Placed: Theo: row 5, column 7 (Workshop); Nora: row 3, column 6 (Gallery).
4. Room counting The victim, Vera, cannot be in the highlighted cells: her room there could not hold exactly one other person. The cells meant: row 1, column 3; row 2, column 4.
5. Shared rows and columns Walter and Mateo can only be in the second column from the left and the third column from the left between them, so nobody else can be in those columns.
6. Room counting The victim, Vera, cannot be in the highlighted cells: her room there could not hold exactly one other person. The cells meant: row 2, column 1. Placed: Vera: row 1, column 4 (Archive); Walter: row 2, column 3 (Archive); Lena: row 4, column 1 (Gift Shop); Mateo: row 7, column 2 (Courtyard).