Do they have to login the first time using Users Widget?
The user come first time into your web site.
Lets say for now that he is into phpBB. The user is loging into phpBB, then he moved into the WordPress as he is browsing your web site. The WP phpBB Bridge plugin check if the user is logged in into phpBB and then create a new session for the same user into WordPress. Finally if the user not exists in WordPress database, the WP phpBB Bridge will generate a new user with the same username as the one in phpBB site.
Is it possible to bridge user deletion too?
The user cannot deleted automaticly from WordPress database.
Does anyone else having problems using this plugin? When I active this plugin all new comments that users leave on my webapge automatically move to last category and are visible as comments on all my posts not just the one being commented.
Us I know until now there is no user with problem similar to yours
is it possible to have users log into the WP site and be automatically singed in to the phpbb forum?
No. This is no possible. This is a WordPress plugin and not a phpBB module, so we don't have the ability to make a lot of functionality into plugin to handle phpBB tasks.
What about transferring existing wordpress users to phpbb?
Is not posible to move WordPress users into phpBB. The main problem of making that move is the password hash. Because both WordPress and phpBB using deferent hashing keys/methods is not posible to transfer that users, while they will need to make a password reset.