Hi,
The iPhone (iOS Device) and the Mac will only Sync from the day that both start using the same App ID or OS X 10.8.2 whichever is the earlier.
The "Sync" as Apple calls it was different before this time.
The "Sync" is only "Display on all Devices" and does not involve anything else (if you delete a message on one device it remains on the other)
The iMessages are "pushed" at the devices that have the Apple ID or iPhone number and when the iMessages has been sent that is it.
It will not re-sync if you remove the iMessage.
I really don't use iMessages on my iPhone enough to even try "Load earlier Messages".
If you have synced (Backup) the iPhone onto the Mac using iTunes it is apparently possible to extract the messages from the Backup.
However I don't sync this way and don't know the names of the apps involved.
9:06 pm Wednesday; February 19, 2014
iMac 2.5Ghz 5i 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad