new widget between the header and the slideshow

instead of:
add_action('__before_main_container' , 'display_my_fp_widget', 15 );

you can use a different hook

add_action('__after_header' , 'display_my_fp_widget', 5 );

hope this helps.

Through all the code 😀
Standard customizr’s pages action hooks

Use __after_header with priority 15 (slider is hooked to __after_header with priority 10)

Hope this helps 😉