We recognize that the older design of the WinForms .NET Languages Add-on isn't able to handle these particular scenarios. I checked and our newer WPF .NET Languages Add-on, which has a much more robust design, does handle all of these scenarios well. And its next maintenance release is also adding C# 6 and VB 14 syntax support. If you need these features today, some customers use our WPF SyntaxEditor in their WinForms apps.
We have wanted to try and backport the newer document APIs and .NET Languages Add-on design back to WinForms from WPF for a long time. While all the non-UI pieces port back without much effort, the problem has been the UI end of things. The UI of the WPF version of SyntaxEditor is implemented in a drastically different way than that of WinForms due to the nature of how WPF rendering is done differently than GDI-based WinForms. Now that our 2017.1 WPF Controls are out, we are hoping to soon revisit trying to make a compatibility layer for the main editor view rendering that would be generic across WinForms, WPF, Silverlight, and UWP and have tailored implementations for each platform. Not having that right now is the main stumbling block for not being able to backport the much more advanced text/parsing APIs of the WPF version.