Show Webform Confirmation message in popup window
Without doing any coding to achieve this you could use a module like Purr Messages and then adjust the style of it's output with CSS, this would apply to all system messages though..

Otherwise like zerocoolhotone mentioned, the simple dialog is an effective way of achieving this too.

Regarding your second question, it's a bug/issue with webform, if memory serves me right you can use the development snapshot to overcome it temporarily until they roll it out.