Hello,
1) Please deactivate all the third-party plugins, activate the parent theme, then clear cache and check back.
If the problem still continues then leave all the third-party plugins deactivated and provide with the admin panel login details of the website with the permissions to debug then we will troubleshoot with the theme plugins.
2) You can find the header banner settings under Theme Settings >> Shop >> Header Banner.
Screenshot for Clarification: https://ibb.co/khDYxKj
3) Please provide some relevant screenshots of what you require to check it myself and help you out accordingly.
Best Regards.