Solution:

After five minutes, the second friend turned around to see if anyone was there or not. At this moment, he was stabbed on back by his friend.

Solution:

Molly

Explanation:
Let us suppose that Patrick was not in the second bedroom. This proposes that Jerry, Molly, Max and Mike are liars. It infers that Patrick and Catherine speak truth. But that is not possible as they do not coincide on Mike's location. Therefore, we are wrong with our assumption and Patrick was in the second bedroom.
So far, we have established the fact that Patrick was in the second bedroom. Additionally, Patrick and Catherine are liars

Now, let us suppose that Jerry speaks truth. This proposes that Max was in the bathroom and Jerry in the second bedroom. This infers that Molly and Max are liars as they told that Jerry was not in the second bedroom. Also it implies that Mike lied, as he said Max was in kitchen. Therefore we have three new liars now, which totals the number of liars to 5, which contradicts the given situation in the question.
Hence, now we have established the fact that Jerry is a liar.

Let us now suppose that Molly speaks truth. This proposes that Jerry was in the living room and Max was in the Kitchen. This infers that Max and Mike are liars. But again, this is contradicting the situation as we have already established the fact that there are again 5 liars (counting above 3 established liars)
Therefore, now we have established that Molly is a liar. Max and Mike speak truth. Catherine was in the bathroom. Jerry was in the first bedroom. Max was in the kitchen. Mike was in second bedroom with Patrick.

The killer has to be Molly as no one else was present in the living room.

Solution:

This is because Adam is a hangman by profession. Apparently, his wife has been sentenced to death and he had to do it with his own hands.

Solution:

Yes, the killer is Samoa. The numbers on the calendar represents the months and the initial of every month reflects the name of the killer.
9 – September (S)
4 – April (A)
5 – May (M)
10 – October (O)
8 – August (A)

Solution:

It is simple. The sheriff never told him where the crime scene was. But still he reached at the right place. This points towards the fact that he knew it already and that he is the killer himself.

Solution:

At every floor, the detective had to open the window himself. If the man had committed suicide, then at least one of the windows must have been open. A person who is committing suicide won’t close the window behind him. Thus he knows that someone pushed him off the edge and closed the window afterwards.

Solution:

The wife booked a two way ticket for herself and booked a one way ticket for her husband.

Solution:

The dead man used an ice knife to commit suicide. The ice held the knife pointing upwards as he dropped on the chair with his back facing the knife. Later, the ice melted leaving moisture around the body.

Solution:

It is quite possible. The poison was not on the drink but in the middle of ice cubes. The first one drank immediately and thus the poison could not reach his body while when the other drank after half an hour, the ice cubes had melted. Thus he died.

Solution:

When you were inside the shed, the murderer replaced the lock with his own one that was identical to your original lock. When you locked the store room, you did not require a key and there was nothing abnormal for you. When you left, the murderer opened the lock with his key, planted the dead body inside and replaced the lock again putting the original lock in the place. He then closed it without any problem.