Use the shorten operator to put a maximum number of character to display. You will have to code the link for the Read More... manually after the display of the description.
>no matter what I changed in line 13, either display the whole distripution or display nothing.
I think that there might be some syntax problems with the data_map calls, but unfortunately, I am not good enough with that to offer any sound advice.
Did you have it working at one point, but with the long version of the description? If so, I would get to that point, and just apply the shorten operator to that.
You would want to link to a new template that you would make that only displays the file description. (create a new template, set the override to display that content with the new template) However, it would probably be better to the a File display template which displays file name, full file description, download link, author, etc.) and make that the page that is linked from the Read More link.