Hi,
So several things here.
1. Single stage checkout is still possible. Just have the login, cart, payment views on the checkout workflow in one step. The address view without the login view on the checkout workflow never worked properly as it is not meant to work properly so I doubt that it was working before on your website and if it was, then it's not normal, it shouldn't have.
2. The blank lines come from the way you have your address_template view files. Go in the menu Display->Views, edit your address_template view files and remove the unecessary tags for your website.
Also, the address selector can be changed with the "address selector" option of the configuration. Before it was "popup" by default. You can always revert to it if you want.