You have two different issues at play here:
1) Addon Manager: you just can't use an older version of it, because the current sceneries requires the version that comes with them, the older ones will not work anymore.
Note that, you might have been to have an older installer to run, because older installers (and all Cloud9 installers) did not checked for FS9.1 to be updated, but then the scenery wouldn't work properly, that's why we check the FS version in the current installers, so people would not install it in FS9.0 by mistake, only to discover the scenery would probably crash randomly because of this.
So, you have to use the current version, that requires 9.1 to be installed.
2) FS9.1 - The updater froze on ABLSCRIPT, but I don't think that file is the issue, it just happening to be the first in alphabetical order of the FS9\MODULES folder that needs to be updated, so the Updater failed immediately.
Having a BACKUP folder or not, should just change the fact that you get a message from the updater that you already have installed it, it shouldn't make it fail so, there's something else going on here.
You might try another solution:
- Manually remove FS9.EXE
- Manually remove all the original Microsoft .DLL modules in the FS9\MODULES folder. They should be quite easy to tell apart from the 3rd party modules, because they all share the same date/time
- Insert your FS9 CD1, run setup, and see if it offers you to "Repair" the installation, it should just restore the file you removed, in their original form.
- Try to run the 9.1 Updater, after having removed the BACKUP folder, if there's any.