First of all inplace merge :
T(n)=2T(n/2) + O(n2)
Using master theorem f(n) = Big Omega(n)
T(n)=O(n2)
Yes,if we use linked lists for merge sort ,
1)finding mid =O(n)
2)solving recursively 2T(n/2)
3)Inplace merge using Linked lists =O(n)
T(n)=2T(n/2) + O(n)
=O(nlgn)