<p>What is the best way to get the TAPI version and additional TAPI information with C#? </p>

Interop.
Use the
lpdwAPIVersion
parameter of the lineInitializeEx function.
http://msdn.microsoft.com/en-us/library/ms735983(VS.85).aspx
"Upon successful completion of this request, this location is filled with the highest API version supported by TAPI, thereby allowing the application to detect and adapt to having been installed on a system with a different version of TAPI."
You're going to have to start plugging away with Interop calls sooner or later when you're programming TAPI.