am I right in thinking the DirectX library only exports a single function and _everything_ else is through DX interfaces

Yup! That's why I didn't have to create a gazillion passthrough functions.

The original DLL in my modern Windows installation has these 8 exports:

    DirectInputCreateA
    DirectInputCreateEx
    DirectInputCreateW
    DllCanUnloadNow
    DllGetClassObject
    DllRegisterServer
    DllUnregisterServer

The game only calls DirectInputCreateA, and the rest happens via the COM object that that function creates.