A little old question but i think someone might find this useful
This is how i solved a similar problem where i wanted to interact with gamepad and keyboard only in a menu with game mode set to UI only, and use B button on an xbox controller as the Accept button, no mouse at all.
On UMG Override the OnKeyDown Function
Get Key is used only for the IsValid & Branch, I noticed it was producing odd behaviour if the Keyboard Menu Key was pressed. Then on the Event Graph I made a custom event (SelectorKey)
What it does is, it gets every button mapping for every action and compares it with the pressed key, then if the key exist within an action mapping, get its ActionName, convert it into a string an use a switch on string, this way i control the path.
The problems i have found to my solution, is that it must be implemented in each UMG and must have keyboard focus and that you cant set speed it does check, so to scroll by holding the button for example, it does it too fast, thats why i used the Is Repeat node, so it only does it once.