I think it was just overly aggressive settings of compression parameters. I don't see any evidence that the jbig2 compressor was implemented incorrectly. Source: [1]
Right. Jbig2 supports lossless compression. I'm not very familiar with the bug, but it could have been a setting somewhere in the scanner/copier that it was changed to lossy compression instead. Or they had lossy compression on by default or misconfigured some other way (probably a bad idea for text documents).
No. The bug was when using the "Scan to PDF" function. It happened on all quality settings. Copying (scanning+printing in one step, no PDF) was not effected.
I remember differently, but I don't want to pull up the source right now.
I did check some of the sources, but was not able to find the one I remember which had statistics on it.
The xerox FAQ to it does lead me to consider that I might be confusing this with some other incident though, as they claim that Scanning is the only thing that is affected.