Hello,
1/ I don’t see problem with the mega menu font https://prnt.sc/26cqre6 It’s the same like you use for all the other mega menus. So, what exactly do you want to change for that? What font family do you want to use for your mega-menus?
2/ This option does not have desktop/mobile modes. So, We would recommend you to use transparent bg for that to suit both headers https://prnt.sc/26cqood
Regards