MySQL Replication - Master Master - Single Master write
With a single write master &lt;> master setup, I don't need to worry about setting the auto_increment_offset correct?


Yes, you are right. I have setup Master-Master setups for years with one Write Master. I never touched the auto_increment options. Never had an incident.


  Does MySQL replicate all databases by default, or do I need to explicitly identify them via binlog-do-db?


Yes, it is by default. Although binlog-do-db exists, I would not recommend it since you are doing Master-Master. If you have additional slaves, it is best to setup filtering on the slave. That way, you have all binlog events to point-in-time recovery and other incidentals. Such incidentals might include Star Topologies (which feature Distribution Masters, a Master that does not house any data, only binlogs Such binlogs could be filter with binlog_do_db to reduce binlog traffic to slaves).


  Do I need to do an export from master1 and import to master2, before I start replication ?


You do need to to export from master1 and import to master2.


  can I start master 2 to pick up all transactions over the last several months?


The only way you can do that is if you had every binlog since you loaded master1 aznd never erased any of the binlogs. Most people rotate their binlogs, making this impossible.

I need to setup a "replicator" user (with replication perms) on both master1 and master2?

Yes

EPILOGUE

I have setup ucarp over the years as the failover mechanism for the floating IP. Most use Linux Heartbeat, Pacemaker, and others. So, you are in the ballpark of what needs to be done.