1. You would need to create such queries and scripts or use functions etc from photopost after requiring our pp-inc.php file
Like calling display_gallery would display a box of photos on the page now of course we are dealing with if thats a php page. However I cant really discuss code modification in product support.
2. If the applications are integrated meaning reviewpost and classifieds integrate with the gallery users table then your integrated and set permissions accordingly.
3. yep must use the htaccess file if you want to use the new
SEO.