Use n-wise reduction to get the sums, then divide:
f←{(⍺+/⍵)÷⍺}
2 f 1 2 3 4 5
1.5 2.5 3.5 4.5
3 f 1 2 3 4 5
2 3 4
4 f 1 2 3 4 5
2.5 3.5
Oops, now with your exact inputs:
f←{(⍺+/⍵)÷⍺}
2 f 2 3 4 5 6
2.5 3.5 4.5 5.5
3 f 2 3 4 5 6
3 4 5
4 f 2 3 4 5 6
3.5 4.5