The excel driver is a handy tool for loading an Excel file to a database.
Please note that it is not designed to be as powerful as a full-featured database connector: no joins, no complex filters, no SQL functions, etc.
The idea is to manipulate your Excel Sheets as a source in a mapping, using simple filters and expressions. And then do the advanced transformations / filters / joins in the Staging Area. In this way, the mapping will will load each excel column only once, even if you have several target fields (your issue#1 will not exist in a mapping context).
That being said, note that we plan to add features to this driver. Including Filter improvements.
I hope this helps.