PKZipFix is highly overrated. It doesn't do much. If I remember right, it simply repairs the Zip file header, so if you have data corruption in the first bytes then it can try to find a data stream and make a valid Zip file again. But if you have a data error in the middle of the compressed data stream, then your out of luck. I might have that slightly wrong: what I do remember clearly is that when I learned what it really does, I was rather disappointed with how little good it was ever likely to do.
I would expect mount to use the wrong filesystem, and need to use mount -t vfat or mount_msdos or something like that. Maybe your operating system is smart enough to detect that.
Having said that, unzippers will often ignore extra data at the start of the file, which they do so that they can work with various self-extracting files. So, maybe some flexibility will work.
There are different formats of how zip files may span. The official standard was that spanning wasn't supported, for some time, and so different implementers extended the standard in different ways.
Be very wary of the zip files: watch the extraction results carefully. I think some more recent efforts have tried to merge things a bit so that there is more compatibility, but if you may be using different software, then know that Zip file spanning isn't necessarily something where all implementations have always been compatible.