For the comments width, try adding the following to your CSS stylesheet:
.fb-comments, .fb-comments iframe[style] {width: 100% !important;}Would love to have your feedback on that. Things to specifically look for are that it does *not* alter the Like button width or any other of the FB social widgets.
For posting to the wall on a post, we just don't have that yet. There's ways to do it with PHP, but Facebook wants to make sure that the user is very aware that the post will be made. Additionally, we have a Javascript call that you could use to present a popup for the user to 'approve' the message, but you'd need to integrate that into the "Submit" action on Kunena.. and make sure that it doesn't prevent the actual post in Kunena from being submitted.
If you'd like to investigate more (and know a little PHP to do some of the investigation on your own), we'd gladly help point you in the right direction for either the automated post (likely the easiest route) or the Javascript route (harder, but more compliant and lets the user have a choice).
Hope that helps,
Alex