Sure delete

I'm using this good utility to definitely erase the free space of my HD. Now
I'd like to ask 2 questions:
1) I'm sure the free space of my HD is unrecoverable, but do existing files
keep any track of the former data recorded in their physical sectors?
Namely, before I ran Sure Delete the free sectors were currently re-used to
record new or modified files, so perhaps they have been recorded only 1,2 or
3 times and so some highlevel recovery utility would be able to recover
their previous content. I guess there would need an utility "normalising"
the voltage level of o's and 1's in existing files in order to definitely
cancel the previous content - maybe this utility exists? Perhaps I could do
this way: I could copy each file and then erase it by Sure Delete, indeed
this is very time consuming but I think it could work. Or perhaps I could
record an iso imagine of my HD, then format and restore it? Does formatting
cancel any track of the previous content?
2) Does anyone know how to run Sure Delete from a line command, by
specifying file(s) to be deleted and number of passes?
TIA
il barbi