Solution:

Number 3

Explanation:
Since it's a ladies bathroom, so only ladies can enter the bathroom and hence most likely the killer.

Solution:

Bond was suspicious of the women as one never knocks its own door at the hotel.

Solution:

Costa

Explanation:
c + 1 characters-> d
o + 2 characters-> q
s + 3 characters-> v
t + 4 characters-> x
a + 5 characters-> f

=> dqvxf = costa

Solution:

Since the knife was in her hand which implies that she was planning to kill her husband "Inspector Montalbano".

Solution:

The numbers that have been written over the calendar are:
12, 4, 8 and 3

A few numbers have been circled under particular months. The curious thing is that the numbers that have been written are the same number of corresponding months under which the dates have been marked.

What we can judge by the overall calendar is that the dates that have been marked may refer to the letters at that position in the month's name. In addition, the written numbers can be the position of those letters. Let's simplify below:

12th month is December
The number marked under December = 5
The fifth letter of December = M

4th month is April
The number marked under April = 4
The fourth letter of April = I

8th month is August
The number marked under August is 6
The sixth letter of August = T

3rd month is March
The numbers marked under March are 4 and 5
The fourth and fifth letter of March = C and H

When you combine the letters, you get the name as MITCH.
Therefore, Mitch is the killer.

Solution:

Maths Teacher

Explanation:
How can she give grades on the 1st day of the college?

Solution:

The murderer is Marcus.

The notes hinted towards the name of Marcus. How?
Just take the first letter from Compound, Art Room, Restroom, Underwater Room, Study Room and Movie Room and you will get:

C, A, R, U, S and M.

Rearranging these words leads to the name Marcus who is the murderer.

Solution:

The murderer is Jack.

Christopher is the seer so exclude him.
Now, if Danny was murderer, then Garry should have been killed.
Now we know that Garry was blocked. Therefore he cannot be the murderer.
Harry can't be the killer as he was killed himself.
This leaves with Jack.

Solution:

The stranger said that he thought the room was his. If so, he must have had the keys to the room and thus he must not have knocked.

Solution:

The Bangladeshi seaman is the thief. This is because the Japanese flag cannot be identified as upside down as it is the same both ways. Thus he is just trying to escape the situation.