If you can code, I suggest creating a custom addin with something like ExcelDNA. You offer an excel function to load the report in memory based on a given date, name or whatever. And a few functions to query that loaded report, that take as parameter the value returned by the load function to identify the report.

So that gives the user the ability to query for a certain date, or sum for a date range, or to get a breakdown by some meta data. Being able to load multiple reports allows them to do deltas. And switching to a newer version of the report just takes changing the argument of the load function and press F9.

Worth also giving the users sample spreadsheet, or having a way to export the full report with all these formulas in Excel so that they can learn by example.

The only thing to watch for is memory clean up. You need somehow to have a way to unload all reports otherwise your memory blows up over time.