Hello,
1), 2) I checked the adding to cart process and see that server response time is very slow, compare your site and our demo http://recordit.co/737jwT5u3q
So, contact your hosting provider and ask to improve the server settings and server response time.
2) I don’t see issues with the sticky header https://recordit.co/OwLjBTjbye
Could you, please, explain in more detail?
Regards