Solution: The Toast in the Green Room at the Theatre

The clues

• 
  BrunoThe cook who tastes everything.

• 
  Jonas · VictimThe shy librarian.

  The victim. He was alone with the killer. He was on a bench.

• 
  IreneThe architect restoring the east wing.

  She was in the third row.

• 
  HugoThe retired colonel.

  He was on a bench.

• 
  DarioThe jazz musician.

  He was beside a piano.

• 
  ElenaThe gallery owner.

  She was beside a plant.

• 
  SofiaThe chess prodigy.

  She was in the Green Room.

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. Pinned suspects Irene can only be in the third row, so nobody else can be there.
2. Shared rows and columns Jonas and Hugo can only be in the top row and the sixth row between them, so nobody else can be in those rows.
3. Pinned suspects Dario can only be in the second row, so nobody else can be there.
4. Pinned suspects Sofia can only be in the bottom row, so nobody else can be there.
5. The only one who fits Only Bruno can still reach the left column, so he must be there. Placed: Bruno: row 5, column 1 (Workshop).
6. Room counting The victim, Jonas, cannot be in the highlighted cells: his room there could not hold exactly one other person. The cells meant: row 6, column 4.
7. Shared rows and columns Jonas, Dario and Sofia can only be in the second column from the left, the third column from the left and the sixth column from the left between them, so nobody else can be in those columns. Placed: Hugo: row 6, column 4 (Cloakroom); Jonas: row 1, column 6 (Corridor); Dario: row 2, column 3 (Prop Room); Sofia: row 7, column 2 (Green Room).
8. Room counting The victim, Jonas, was in the Corridor with exactly one other person. Only Irene can still get there. Placed: Irene: row 3, column 7 (Corridor); Elena: row 4, column 5 (Foyer).