Very interesting write up. Seems very hard to find good master-master setup instructions for Drupal. A few things made me wonder though.
Why keep cache_* in the database at all? It would be easy to push them to memcache ( http://drupal.org/project/memcache ). You can do the same for sessions (either memcache or mongodb).
The Drupal mongodb module ( http://drupal.org/project/mongodb ) also supports a locking mechanism (D6 patch though), so that would be taken case aswell. And about syncing /files/ .. what about mounting the same harddrive?
Was this writeup made for Drupal 6 or Drupal 7?