My main problem is the appearance of slugs in the pages URL (because of the pages hierarchy). The reason for removing the slugs is SEO.
After digging a while I figured out I should move ALL of my 300 pages to posts, and use WP No Category Base plugin to remove the category.
I found out I can use an SQL query to set all page as posts, by using:
UPDATE wordpress.wp_posts SET post_type = 'posts' WHERE post_type = 'page';
after running the query all page disappeared, but did NOT appeared as posts (nothing there)!
Please help with my problems...