VirtueMart unfortunately, requires a complete profile to be filled out before we can create the user in their system and add them to a shopping group. This means that all the required fields have a real value. We're working on some ways to improve the system as the VirtueMart plugin that's available is the first release we've put out. As for the form being shorter, you can opt to show additional fields, or hide fields that are prefilled in from Facebook. These options allow you to short the registration form considerably over the default VirtueMart form. It's just a matter of how you want it to show up.
With that said, we are looking into a way to do the single-click registration with the VirtueMart plugin enabled, but again, the user's VM required fields have to be filled out, which complicates matters.
There's no way with JFBConnect to use a pop-up for additional information.
Finally, for the {JFBCComments} tag, yes, it doesn't matter how you include the comment box (content plugin, module, tag), the emails should be sent on any new comments.
Hope that helps, but if you have other questions, just let me know!
Alex