1.If you take the inverse of the secret matrix and apply it to the message vector starting at the begining. Apply the matrix row by row. Then replace the numbers in the final vector with there corisponding letter
2. No, but she can find the matrix for the input and output. If she knows the input then she does not need to decrypt it.
3. 1 0 4 2 ? 0 0 1 0 => ? 1 ? 1 0 1 0 ? 3
