sortable columns on a custom post type won’t work

i believe the right hook for displaying the actual column content (each line) is

manage_{$post_type}_posts_custom_column

so change in your code:

add_action( 'manage_event_custom_column', 'event_date_column_display', 10, 2 );

to:

add_action( 'manage_event_posts_custom_column', 'event_date_column_display', 10, 2 );