About the first question, that table always is a MyISAM table, and it was chosen for efficiency. So even if you pick InnoDB during the installation, that table will be a MyISAM table.
About the second question, we implemented support for PDO MySQL because, according to some articles we found on the web, it was supposed to be more efficient than the mysql extension. I've done some measurements in Ubuntu and there wasn't any improvements in performance. Maybe because of how we had to use PDO MySQL to implement a data access interface, or maybe in other platforms there's a difference, I don't know, but we left it in the rare case that a user doesn't have the mysql extension but does have the PDO extension.