In just two simple steps you can remove the Login or continue as guest pane and simplify your checkout process. In my opinion, this step is completely unnecessary and just confuses your customers.
As of writing this article, there is no module that allows you to create a Facebook Product Catalog in combination with Drupal 8 and Drupal Commerce 2.x. That's why I'm open-sourcing my custom solution today.
Implementing Out of Stock feature in Drupal Commerce 2.x is easy. Unfortunately, you can't just enable this feature, because Commerce doesn't support it out of the box. You have to get your hands dirty and write some code.