1) The MySQLi server behaviors are a recreation of the standard built in server behaviors that are shipped with DW modified to use the MySQLi library.
Data Bridge is a collection of more advanced tools. At this time, Data bridge does not support the MySQLi library, but we are working on an upgrade for that. The tools in Data Bridge include Data Assist, which is a wizard to create advanced Insert, update, details, search and results page sets for your tables. It can be used to add File Uploaded behaviors to those page, or Rich HTML Editor fields.
It also contains the Security Assist wizard which can be used to create user regression / login pages for your site.
a full listing of the tools in Data Bridge can be found here:
2) Yes, Data Bridge is supported on Dreamweaver CS4 and above.
3) Yes, you can buy super Suite later, but it would be cheaper to buy Super Suite at the start.
4) The MySQLi extension uses the Object Oriented Syntax, when Data Bridge is upgraded to use the MySQLi library, it will also use the object oriented syntax.
5) We don't plan to implement PDO.