The database error you have posted in the first post does not seem to originate from the uninstall script, so not really related to your uninstall problem:
Script :
http://www.sevenstring.org/forum/showthread.php?t=48436
I don't see any major errors in the uninstall script. My guess is that the only error made, is that you thought the script didn't work and canceled it halfway.
If you have large tables, then these ALTER TABLEs can take a long time. Best is to uninstall again, if an error, edit the uninstall code removing those tasks that are already completed from the top, retrying after each change until it runs till the end.