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In retrospect, I obviously should have done my transpose earlier on to avoid the multiple multirow tools. My answer doesn't quite match the output, but I believe that is just because of different months chosen for the running averages.
My solution included some inefficiency with several multi-row formulas, but that's just how I wanted to do it at the time. Also, I don't think the solution file provided for the challenge is doing what it is supposed to be doing.