"the father remarried and the wife has a biological child. That child did not show up on the chart. He is not a direct descendant. Is this why? Is there a way to force it?"
It is because the child is not a descendant in the database.
Link the step child to the step parent, and label the relationship like I described before.
"in the descendant report can the word 'unknown' be avoided all together?"
Check the report settings to make sure you haven't missed something there. Otherwise, the only way I know of to get rid of unwanted unknowns is to save the report in an editable format (rtf, txt) and edit it.
Hope this helps. :-)