I would like to figure out how to create a C# Remote Desktop client and the documentation on the internet varies from sparse to non-existent. Or if you have this information
The documentation for the advanced settings can be found here: IMsRdpClientAdvancedSettings interface
An extensive implementation of RDP is available on github (RemoteNG on github). Looking there is probably the best way to solve own implementation problems. Because mRemote is an implementation for several protocols (RDP, VNC, Citrix and others) it takes some time to understand what they are doing.
Another, simplier implementation can be found here: Mulit RDP client .NET. The disadvantage is that this solution is over 10 years old, so a fairly old version of the OCX is implemented there. The big advantage is that, unlike mRemote, this code is not published under the GPL.
MSDN has documentation for the Remote Desktop ActiveX Control Interface
(What a long and descriptive name :) )