I am no expert on styles, but doesn't the "bitfield" in the style.cfg choose which standard bbcodes are hardcoded in includes/bbcode.php and which are in bbcode.html?
I noticed your style is using a different bitfield and has no bbcode.html, so it would be using prosilver's.
You should be able to use a better Q&A to prevent those registrations.
If you want to leave registration disabled, you can use the ACP Add User extension to add the users without having to enable registration.
Nope. It is a user permission.
Go to Permissions -> Group permissions
Select Registered Users
Select type: User Permissions
Click "Advanced Permissions"
Private messages tab
Set "Can send messages to multiple users to "No"
Set "Can send messages to Groups" to "No"
Click "Apply permissions"