Hello,
1) content-product-slider.php can be changed using child theme – https://developer.wordpress.org/themes/advanced-topics/child-themes/
2,3) secondary_menu.php and walkers.php – can’t be edited via the child theme. Describe in more detail why do you want to edit these files, maybe we will be able to give some suggestions.
4) Use Insert Headers and Footers plugin.
Regards















