average & sum using index & match formulas

I am getting used to using the index & match functions so I'm hoping someone can help me.

on the attached spreadsheet in the cells highlighted in yellow on the "team rankings" tab. I need to get the sum of the corresponding information from the "weekly team stats" tab. For each team member for the month indicated on that table. For the cells in blue I need the same but I need the average instead of sum.

For the last table the cells in green need to be the sum of all the calls handled for that team member for everything on the "weekly team stats" tab. and the pink cells need to be the average of all corresponding data from the "weekly team stats" tab.

Not exactly what I am looking for. I want to use a sum or average function with an index match so that if the header name changes it will still work without having to make any changes to the formulas. I'm just not sure how to use those together.