Currently, we do not have profile integration with VirtueMart. It's in development, and we're hoping to have integration with it by the end of this month or early next month.
Once done, it will allow you to pull information from their Facebook profile into Virtuemart, add them to a shopper group, and then they can proceed to checkout.
As for the other questions:
1) It depends on what information you need. There's a lot you can pull from Facebook, but depending on your requirements, you may, or may not need to ask for more information.
2) Facebook users have to approve an 'application' you create through Facebook. They will become users of this application, but they will have to specifically choose to become a fan of your page. That's not automatic (and can't be made to be so, per Facebook policy.
3) Their email will be pulled into Joomla, but you'd need to add this to the newsletter. We don't currently have integration with AcyMailing, but if it can read from the Joomla user table, you should be good to go.
4) You can easily configure the Comment box to be page-specific
5) I'm not sure what you mean about a non-commercial sidebar, but we work with a wide-range of extensions. If you're using other Facebook features, you should be able to replace them with JFBConnect.. or continue to use them with little or no modification.
Hope this helps!
JFBConnect and Virtuemart
