Well when you’re going to set blog page header you’ve to set it from the theme option, because blog page don’t take options from page settings. In that case you’ve to set things from theme option. Checkout the screenshot: http://prntscr.com/mkv5f2. By this you’ll be able to set header image in blog page.
And about the error in single event page it’s happening because you didn’t put any performer information in your event settings for which the error is reporting. Please check the screenshot: http://prntscr.com/mkv69b
What happens is that if there’s no data in performer settings there will be errors anyway I’ve fixed it into your theme. We’ll put a patch with the latest updates in the theme.
Please do let us know if you need anymore help or not.