I'm the author of the Atomineer Pro Documentation addin, which would be worth a look. It is focussed more on the source code than externally generated documentation, so it may or may not be the right tool for your needs, but here's a gist of how it relates to your list of specific needs:
Pros: It will help you generate the original comments faster and better than most (if not all) other addins out there, and supports more source code languages and comment formats than other addins. When updating comments, it will go further than other addins to give you control over the formatting (whitespace control both inside and outside the comments, comment-block formats, very good control on word wrapping, entry formatting, indentation, and block separator lines, and support for bulleted lists, etc). It also includes a documentation viewer window that allows you to see how the final documentation could look (depending on how you export it to a rich text form). Live typing aids make editing doc comments much faster and easier.
Cons: It does not (yet) offer rich text editing of the documentation, nor give an overview of docs for a solution. You may find a product like DocumentX might suit your needs better in this area.
