I don't think that we are gaining much by caching these reaggTypes and then looking them up by column name.
For one thing, it needlessly caches nulls for the group by columns, for another thing, it caches reaggtypes for columns that are not ever used in the query.
It would be simpler and probably fast enough just to take each member of mvNewScanColumns in the next loop that isn't in the groupbycolumns and look it up serially in mvColumnArray to find its aggtype/reaggtype. You could even skip over all the groupbycolumns when indexing through mvColumnArray, since it is in column index order.