Most of what you ask is about ASP / ASP.NET, not about Manifold. With that:
2 - Which languages you can use in <%@ ... language=... %> depends on system configuration, mostly it's just the standard languages available as COM / .NET engines on Windows systems, the same ones we support for scripts in Manifold.
3 - ASP uses COM languages, ASP.NET uses .NET languages, you are mixing the two (VBScript is COM and C# is .NET).
4 - Yes, for ASP.NET (not just for C#, for all .NET languages), server-side code is compiled. That makes it faster than in ASP where server-side code is interpreted.
5 - Yes, when you are using Visual Studio to develop applications that use Manifold objects, it makes total sense to reference the Manifold DLL with those objects.
6 - Which editor supports syntax highlighting and other things for VBScript sections in ASP? Visual Studio. Intellisense in ASP / ASP.NET is tricky because the file is split into sections with different sections using different languages, Visual Studio does a pretty good job of dealing with all that (although it's far from perfect).
7 - Don't understand this.