Solution
The NI Modular Instruments Python API is an Open-Source project available in the GitHub repository,
nimi-python.
The ability to select the digital edge trigger is not exposed in the NI-DCPower Python API. The edge type is Rising by default.
This property is not exposed because there is no obvious use case for it and to keep the API leaner.
If there is a critical need for it, the additional functionality can easily be added in the Python and built locally.