It would be very cool if the preview window allowed you to switch between installed browsers. I have opera, sea monkey, chrome, IE, Safari and Firefox installed on my system for testing and it would be great if we could pick between browsers. It would be even better if we could dynamically change between browsers without restarting the plugin.

It would indeed be cool if that were possible, but it isn't.  Other browsers than IE don't provide a way of embedding the browser into other programs.

It is possible to embed a different browser component using Chrome's browser engine, but that would require quite a bit of work. There also used to be a Mozilla browser component, but that looked like being a lot more work than even Chrome, and I haven't seen or heard anything about that in a few years, so it might no longer be maintained.
And in the end, it wouldn't be the same as the installed browser; each version of the plugin would have to package its own version of each browser.