Hello,
First of all, you create a menu as you usually do in WordPress. Next, you check and make sure you have activated in the screen options the options as shown: http://prntscr.com/j0u4wv
Then enter menu item which you want to add mega menu: http://prntscr.com/j0u60x You create HTML block in advance using WP Bakery Page builder. You can take Product grid and set the carousel mode http://prntscr.com/j0u96m
HTML block is created here http://prntscr.com/j0u9pl
As for footer. Footer content is edited in Appearance > Widgets > Footer Columns. Payment methods are the functionality of WooCommerce our theme does not influence on it.
If you have any questions contact.
Best Regards