Still working on that project 10 years later? Regardless I think I know what's going wrong for you and what might be for others in the future.
So like you said: in the data editor go to view tab and de-select "combine structure values" and select "view raw data". Find the unit you want to have a submenu that doesn't revert to the uhhh... main menu/command card after every button press. Of course make sure the unit container is selected not the abilities, buttons etc and type in "sub" into the search bar. You should see these 2 lines among others: