With Common UI, you probably don’t want to be explicitly setting the input mode.
Instead, you should use Common UI itself to manage the input mode.
This plugin is still experimental and as of 5.1 it requires a custom engine to do much to change its default behavior.
I understand updates are in place for 5.2 to make it easier to work with. Until then unless you are comfortable with C++ and custom engines, in 5.1 the control over how the mouse is used is limited.