John, this solution definitely gets rid of the 'show more' text but I think the issue is that guests cannot see the rest of the posts in a particular thread beyond the first one - this is a problem I have been trying to fix the last few weeks as well. An example might be the easiest way to explain:
With a guest log-in the default landing page when you click on the single activity forum is this page: http://bostnet.org/slc/course/view.php?id=2
For the first post "Activity modification for tiered learning" you can see my initial post but not any of the replies, unless you click the "Read the rest of this topic". For the second topic listed, "Success Stories" there is no way to see replies since the post is not long enough to trigger the "Read the rest of this topic" link. I cannot find a setting (site, plugin, user rights) that fixes this; the most promising solution seemed to be to create roles but I couldn't make that work (https://docs.moodle.org/29/en/Forum_poster_role).
Ideally I'd like the landing page to have the layout in this view (which solves the problem): http://bostnet.org/slc/mod/forum/view.php?id=1
IMO, this is a better layout in general for my site even though you do not see the short description, so if there was a way to set this as the default I would be very happy.