indexed-repeat is a "convenience wrapper" for accessing specific questions at a particular repeat index. In this case, it looks like you just want to access the station in the current repeat instance so I think it would be more readable without indexed-repeat. It also would be less work for the engine because it could use the local context of the repeat instance.
Your first try at Get custom data from choices sheet for calculation - #12 by mathieubossaert with name=${station} looks the best approach to me. Could you please try it again as one expression with cases for each month?
I know we had some issues with XLSForm conversion of this kind of expression at some point so maybe that's what you ran into. I thought they would have been fixed by March 2021 but possibly not.