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:

Can be found on the pic below.

Solution:

It can be done as shown in the picture below.

Solution:

We are deck Of Cards

Solution:

It can be done in image below.

Solution:

Circle Under the Eyes.

Solution:

Yellow switch

The trick to solve this problem is to start from the bulb.

Solution:

On the faces of the first dice, you can write 0, 1, 2, 3, 5 and 7
On the faces of the second dice, you can write 0, 1, 2, 4, 6 and 8.

It is simple to crack. Basically, we need 1 and 2 on both the dices to display 11 and 22. Also, we need 0 on both the dices to display dates from 01 to 09.

Now we have taken every number on either dice except. This is the tricky part. Whenever you want to use a 9, you can simply rotate the face with 6 180 degrees.

Solution:

19(snake) + 1(Echidna) + 80(Bird) Egg =>100 eggs.
Cost:
19*5 + 1 + 4 =>100 pence

Solution:

We now have a square of 3x3 size, 2 squares of 2x2 size and 5 squares of 1x1 size.