Hey J,
1. Regarding the problem with your third-party plugin (VikBooking) widget, I see the conflict. This conflict is best investigated by the VikBooking authors because they might not be supporting the method that the Widget Elements use to output widgets.
The widget works fine in the sidebar.

The difference is the Widget Elements use output buffering. Please contact the VikBooking support to see if they support that method.
For your and the VikBooking author’s reference, below are the directories and files of the Widget Area and the Classic Widget Area elements.
- \wp-content\themes\pro\cornerstone\includes\elements\definitions\widget-area.php
- \wp-content\themes\pro\cornerstone\includes\elements\classic_alternate\widget-area.php
- \wp-content\themes\pro\cornerstone\includes\shortcodes\widget-area.php
2. Regarding the Inline Menu, are you referring to this header in the screenshot below?

You need to activate the laptop and the tablet view also. The 13 inch Macbook might be in the range of laptop screen size.

If that’s not it, please tell us what header.
3. Regarding the line, it’s because of the 49.5px Margin Top of the Initial Menu bar. Changing it to 49.4px or below would fix the issue.



Hope that helps.