Hi 
Sorry no-one has answered you yet! I've not much experience with
converting to html. I tend to write all the html coding i need myself
to try to keep it as simple and elegant as reasonably possible,
although my style might be a tad quirky.
Each of the different formats you convert from has it's own strengths
and weaknesses. Probably the best results come from converting Odts?
.Txt doesn't retain any formatting so it's no surprise those go wonky.
.Doc is the generic format currently used by the greatest range of
programs but it's a bit old. It's replacement is DocX which works on
the smallest range of programs.
The squares instead of any character (in this case dashes) is usually
due to limitations of the font chosen. Somewhere a non-standard font
is being used and i'm not sure how to embedd the right font into an
html page. I'm not even sure it's possible. Usually people use css
(cascading style sheets) to define whjat font to use and which to use
if that is not available on a target computer.
Sorry this hasn't been much help! 
Regards from
Tom 