I don't claim to be an "expert", by any means but I can help point you to info that will answer your fifth and sixth questions. Thomas has a nice tutorial over at Polyloop on using the surface replicator to spread mass amounts of replicated trees over a landscape. http://forums.polyloop.net/carrara-tutorials/4317-c5-surface-replicator-dream-island.html

Mark Bremmer discussed that particular problem with displacement mapping in the following thread- http://www.renderosity.com/mod/forumpro/showthread.php?message_id=2600184&ebot_calc_page#message_2600184