What does this rebus puzzle means ?

If we tell you that there is a relation between the numbers and letters in the given figure, can you analyze it and find the missing letter in the last box?

You board a train. The train will have to enter a tunnel soon. You are claustrophobic. Which place is the best for you to sit?

John named his first son Alpha, second Beta, third Charlie and fourth Delta. What is the fifth son's name?

A man was driving a black truck. His lights were not on. The moon was not out. A lady was crossing the street. How did the man see her?

When my father was 31 I was 8. Now he is twice as old as me. How old am I?

Find a 9-digit number, which you will gradually round off starting with units, then tenth, hundred etc., until you get to the last numeral, which you do not round off. The rounding alternates (up, down, up ...). After rounding off 8 times, the final number is 500000000. The original number is commensurable by 6 and 7, all the numbers from 1 to 9 are used, and after rounding four times the sum of the not-rounded numerals equals 24.

It is always ahead of me yet I can never see it. What is it?

Make a sentence consisting of three words that have the whole alphabet.

A word I know,
Six letters contain,
Subtract just one,
And twelve is what remains.