If your account is closed due to late payment, you will receive warning emails few weeks before the due date and you will have plenty of time to backup your servers.
If your account is closed because you break the TOS, you will not have a chance to log into your account any more.
It is my understanding that you want to give your brother access to upload files to your domain. If so, you only need to create a FTP account for your brother.
Yes, that is all you need to pay.
It is all explained on the TOS page. If you are not sure, always ask DH support first.
Don't forget that you are on a shared server. A website like facebook is database entensive. It will survive at the beginning but will definitely suffer performance issue when it grows. A VPS or dedicated server will be your choice when your site is mature.