Don't believe that is going to be possible, sorting is either all or nothing, sorry.
Once you've caught up a bit, you won't need to check out the first post quite so often, although I sometimes do if I've forgotten what started the thread!
When I need to do it, I read the "new" posts first, then check out the original post(s). You can use the page numbers on the top of the posts in each thread to go to page 1, or the last page, depending on which way you've got it sorted, then use the Home or End keys to go to the first post, again depending on how you've got them sorted. (I keep new posts at the top, so I would use the last page number (one click) and then "End" (a key stroke) and a "page up" (a key stroke) to get to the first post, since "End" takes me all the way to the bottom of the page.
Then I would use the "back" button on my browser to go back to where I started with the new posts.
Hope this helps, because I don't think we can do the other, and we appreciate you being an active KindleBoards member!
Betsy