I believe you can't upgrade the OS unless you delete Vendor.xml, unless the vendor that released the OS is the vendor of your phone...so if you didn't delete it and you still upgraded, that's not an issue.
You can do the upgrade w/any version of DM...4.2.2 is not required.
You can uninstall DM 4.6 and download and install DM 4.5...I've seen posts that 4.6 may be problemmatic. I've had no trouble syncing w/DM 4.5.
And you don't have to use Media Manager - you can copy music/files to your SD card directly.
If the OS is running OK on your Curve, no reason to undo that - leave well enough alone.