Not sure that I understand your idea right, so let me explain what I have understood:

You would like to have an additional control which displays only the last x comments (equal the most recent entries, but only for comments), is that what you mean ?

Your second suggestion sounds for me not like a very good idea, let me explain why not:

In case of your blog settings users and anonymous users can add comments to your blog entries. In a simple text box it is quiet simple to disallow javascripts and other code which can have security issues for a web site. If I use a rich text editor, it is much more difficult to control the user input from.

That is the reason, why I don't like to use a rich text editor for comments.

regards

HP