Loved your guitar improv!
About clenching your jaw and thinking that opening it more both hotizontally and vertically, there is a thread about the tongue that also gets into whether or not you need to open the jaw more to get more sound or more nuance of sound.
It's on page #2 of that thread. http://www.punbb-hosting.com/forums/themodernvocalist/viewtopic.php?id=1887&p=2
There may be some other posts about this topic too, but I do not know where they are. Maybe someone else on this thread will know.
Everything is a balance.
Keeping the energy between your guitar technique and your vocal technique is not an easy thing to achieve, but you are doing a great job learning how to do it!
