Hello,
2/ Check now.
3/ I have checked your product in Safari and don’t see any problem with the fixed image https://gyazo.com/85391376d9d9c36be788db6cbf30a278
Could you, please, provide us with link to product where you have a problem?
4) There is no option to use one static block for the desktop and another one for the mobile. You can just use WP Bakery column responsive settings, create columns with the separate content for the desktop and mobile and choose to hide column for the desktop but show to mobile etc https://www.awesomescreenshot.com/image/24307760?key=c151dd2b596cd6c3dbd0b090118947b5
Regards