Okay, you've got the right idea, but let me summarize
1. If the DLL has a standard C interface (Win32, built with C/C++, etc), then use Call Library Node
2. If the DLL is COM/COM+/ActiveX, use the ActiveX Palette
3. If the DLL is .NET, use the .NET Palette
As to updating the list of methods - the issue has to do with a tradeoff of performance and usability. When you pick a .NET assembly, we load it in, grab the information from it and cache it. We could redo that every time you select it, but that's pretty expensive. Also, if you change the .NET assembly at all, we would need to recompile the VI just in case any method signature changed (add or remove a parameter, etc).
All this means that it is very non-trivial to support such a thing. Now, that doesn't mean we aren't looking at it, just that I beg your patience while we get all the issues worked out (you'd be surprised how many "okay, but what if the user does XYZ and then ABC?" you have to go through to avoid corruptions, crashes, and confused users 🙂 ).