Take our apologies for the delay in answering.
Your cache plugin does not support a separate cache for the mobile devices, that’s why you have a problem with the mobile header because it displays the desktop version instead of the mobile version. So, keep the Mobile Content Optimization option disabled in this case (Theme Options > General) or choose another cache plugin that has the option for the cache of the mobile device.
Unfortunately, we need more time to find the reason of the header builder and single product builder issue. Could you, please, provide us with the server error logs? Don’t you have there errors about the memory limit?