1. Use of WorkingMemoryFileLogger seems to be dedicated to StatefulSession - there's no way to keep the same log file for multiple StatelessSession invocations.
Correct, there is no reason why this needs to be like this, improvements to make this useful in more scenario's are welcome.

2. Drools Eclipse plugin provides pretty nice Audit View which reads the generated audit log file. The log file is in fact a serialized java.util.ArrayList in form of xml. Problem is that being a well formed xml this log file does not allow simply append new event to the end: to keep it well formed xml I have to either completely re-write it each time a new event is added or complicate the procedure with read the closing tag </List> to insert the new event before. But even re-writing does not solve the problem: the hierarchy of WorkingMemoryLogger does not allow add it to multiple StatelessSession 's as a listener (see the constructor).
This has been improved for the Drools5 release: the audit log now is a sequence of event objects.  This means it should be possible to simply append events to the already existing log.