I found another reason for number 2 to be 7

it deals with pairs adding up to 9

in each half, you have an inside out pairing
Code:
4 5 6 3 4 5      2 3 4 5 6 ?

4             5      2             ?
  5        4            3       6
     6  3                  4  5
each pairing adds up to 9 so logically the question mark should be 7