Hi Samson – Point 2 – You can change the font size in the Customiser > Typography > Primary Navigation.
Elementor works fine in GP so either your doing something wrong or their is possibly another plugin causing the issue. Do you have any other plugins running? If so disable all of them and see if the problem still exists. You can then eliminate the plugin thats causing the problem by adding one at a time.
Might be worth you posting a link or a screenshot for GP support to see when they wake up ๐