I'm not entirely sure how to answer all of your questions but here goes:
1) the mutiple password thing is easy... all you have to do is when a user enters a password and hits submit then simply check against your database if the password given is valid, if true then direct him to the main page else display the password login for again..
2) For header("Location : index.php"); to work you cannot "send" any output to the screen before using the header function... so what you can do is start your page off with php code that validates the user, if it's a valid login THEN start your tags else call the header function you mentioned in your question...
I hope that helps.