First question: your mailstore was not any where near 16GB when you started this was it?
Because databases tend to expand during the update process and Exchange standard only mounts mailstores up to 16gb before failing. (14GB or 15GB might be near enough) If SP2 completed the default limit is 18GB I guess – but did it complete?
Second question: have you checked that all Exchange services are running now? In particular is ExIFS running?
“The Exchange Installable File System (ExIFS) represents the drive interface installed with Exchange 2003. It enables Exchange to read and write to and from folders.”
Similarly is IIS working and is the Exchange part still coorectly configured? All data moves through IIS to ExIFS to the Mail Store and back. So maybe your install error is actually a IIS end failure.
you might want to check MS05-019 if you didn’t have that on first.
Also did you check if the AD was cleaned off across all domain controllers when the IMF was removed?
I see notes that SP2 wants to create its own from scratch.