Hi Marie:
1. As far as I know, the Apple Sync Services (the process used by OS X to sync data between the computer and external devices) does not allow to overwrite the local information with the data from the handheld. It allows to synchronize and to overwrite handheld data with the computer data. You wil have to be careful to not to select the latter, because you'd wipe your clie's data.
So, you will have to set the appropiate conduits (usually Contacts, Events and Taks) to synchronize.
These are a couple of screenshots for The Missing Sync (just for reference):
http://mac.softpedia.com/screenshots/19-2373_3.png
http://mac.softpedia.com/screenshots/19-2373_1.png
2. I don't know if your clie will be able to sync normally to the Windows PC again. I suspect there is no reason to not work, because both The Missing Sync and the Palm OS for Clie (PC) are only intended to comunicate with the device, retrive data from it, and send data to it, and they should not alter its OS, file promats, etc. But as I said previously, I don't know what will happen. You will have to backup and try, I guess.
Good Luck.
MacMini G4 1.25GHz 1GB Mac OS X (10.4.9)