> After the HTT has completed a site part (HTT showing that 
> site mirroring is finished), which files/ folders are 
> completely deletable ?
You can delete hts-log.txt (logfiles).

If you do not want to update the site in the future, you 
can delete the hts-cache foler (and all its contents).

The top index.html, fade.gif and backblue.gif are only used 
by the top index, which is a redirect to the site top page. 
(for example, a link to www.foo.com/default.html) ; you can 
delete them if you do not want to use the top index;html 
file.