MySQL: Use index in background query
Well, maybe it's not the answer I thought I'd find, but it works perfectly.

I have splitted audit_insert field in another one, audit_insert_datetype, of DATE type. This field has a new index too.

I have changed the query to execute with this field, and I have tried to force the @... variables to be date type (with current_date and date).

The results: the new index is used and the execution time is dramatically reduced.
Maybe it's bad style, but it works as I need.