Webi isn't an ETL tool, it's a reporting tool.
The real solution is to establish the reporting requirements and build some Webi reports with drill/input control functionality and similar but that probably isn't what you wanted to hear.
If the users are messing around in Excel and want to pull everything out of MM, isn't it time to consider some properly defined Bex queries rather than one giant data dump?
In the first instance, establish which columns can be chopped because they are duplicates/not needed and see what you are left with and if that's manageable.