how can I mount a ab backup file from windows? And would I be able to point an undelete program on it?
Android backup files created via adb backup are no images, they are more comparable to ZIP files. Only existing files are included into the backup file. Deleted files can not be included in such an Android backup.
Therefore you will never be able to run an undelete program on an adb backup file.
The only backup files that would make sense to use an undelete program on are NANDroid backup files as those backups files contain one or more whole partition(s).