> -download a discussion forum with comments by user X
if the forum supports a listing of all threads/postings with user X, then pick
the user-relevant part of the url and include it in the httrack command,
exclude the rest.

httrack looks at file formats and urls and not at the content of a webpage,
e.g. downloading only pages containing the words "apple is crap" is not
possible.

-the forum requires login first (username and password)

sorry, but: rtfm & search the forum!

-index the results by username, discussion title

don't know. i'd guess not. try the different index options in the manpage like
-%I (make  an  searchable  index  for  this mirror) and look what happens -
trial an error.