Index sum and strip my matrix

MATL, 36 34 bytes

tnq?`t&+stn:*sytn2/)+ 7M(6Lt3$)tnq

Input is a 2D array with ; as row separator

Try it online! Or verify all test cases.

Explanation

tnq       % Take input. Duplicate, get number of elements, subtract 1
?         % If greater than 0
  `       %   Do...while
    t     %     Duplicate
    &+    %     Sum matrix with its transpose
    s     %     Sum each column. Gives a row vector
    tn:   %     Vector [1 2 ...] with the same size
    *     %     Multiply element-wise
    s     %     Sum of vector. This will be added to center entry of the matrix
    y     %     Duplicate matrix
    tn2/  %     Duplicate, get half its number of elements. Gives non-integer value
    )     %     Get center entry of the matrix, using linear index with implicit rounding
    +     %     Add center entry to sum of previous vector
    7M    %     Push index of center entry again
    (     %     Assgined new value to center of the matrix
    6Lt   %     Array [2 j1-1], twice. This will be used to remove shell
    3$)   %     Apply row and col indices to remove outer shell of the matrix
    tnq   %     Duplicate, number of elements, subtract 1. Falsy if matrix has 1 entry
          %   End do...while implicitly. The loop is exited when matrix has 1 entry
          % End if implicitly
          % Display stack implicitly

Python 2.7, 229 bytes

This is my first attempt at something like this, so hopefully I followed all the rules with this submission. This is just a function which takes in a list of lists as its parameter. I feel like the sums and list comprehension could probably be shortened a little bit, but it was too hard for me. :D

def r(M):
  t=len(M)
  if t==1:return M[0][0]
  M[t/2][t/2]+=sum(a*b for k in [[l[x] for l in M]for x in range(0,t)]for a,b in enumerate(k,1))+sum([i*j for l in M for i,j in enumerate(l,1)])
  return r([p[+1:-1]for p in M[1:-1]])

Thx to Easterly Irk for helping me shave off a few bytes.

C#, 257 bytes

here is a non esolang answer

void f(int[][]p){while(p.Length>1){int a=p.Length;int r=0;for(int i=0;i<a;i++)for(int j=0;j<a;j++)r+=(i+j+2)*p[i][j];p[a/2][a/2]+=r;p=p.Where((i,n)=>n>0&&n<p.Length-1).Select(k=>k.Where((i,n)=>n>0&&n<p.Length-1).ToArray()).ToArray();}Console.Write(p[0][0]);

ungolfed:

void f(int[][]p)
    {
        while (p.Length>1)
        {
            int a=p.Length;
            int r=0; //integer for number to add to middle
            for (int i = 0; i < a; i++)
                for (int j = 0; j < a; j++)
                    r +=(i+j+2)*p[i][j]; //add each element to counter according to their 1 based index
            p[a / 2][a / 2] += r; //add counter to middle
            p = p.Where((i, n) => n > 0 && n < p.Length - 1).Select(k => k.Where((i, n) => n > 0 && n < p.Length - 1).ToArray()).ToArray(); //strip outer shell from array
        }
        Console.Write(p[0][0]); //print last and only value in array
    }