> 4. I used CTRL+C and httrack said "Finishing pending
> transfers... press again ^C to quit." and I left it
> to complete, which took a while

Ok, this is why the HTML files have been written with many external links
actually - httrack quickly finished to dump all html pages pending, rewriting
links not downloaded as external ones.

> Any idea how the ZIP file got corrupted, as I have
> left httrack to gracefully quit?
The ZIP file might not be corrupted actually ; the data being downloaded again
(if using the "continue" switch) is probably really missing (what is the size
of the ZIP file ?)