@Ed: For most of the question first please check the Theme Instruction page http://catchthemes.com/theme-instructions/catch-mustang/
You can add any page, post, gallery from Custom Menu. Go to “Appearance => Menus” and build custom menu when you can just select the page, post, gallery and categories you want in menu and then add to menu. See this for more details about custom menu http://en.support.wordpress.com/menus/
I don’t see that you have added “Catchthemes: Featured Gallery” widget in sidebar.
Yes, the gallery page might be conflicting. You should change or delete it.
You can add your gallery page in Custom menu as explain in above paragraph.
Yes, it’ bit difficult to jump to WordPress theme directly from HTML/CSS. But I will try to help you as much as I can.