I recall one scene of the game:
The apprentice is pushed of a cliff, and he asks:
"You were never planning to overthrow the Emperor!"
"Not with you, no."
This supports your theory. He saw the power in the boy, planned to overthrow, but when Palpatine found out he had to switch motives. And decided to 'use' Galen and wait for another apprentice, stronger than this one. (Luke)
"I think the Devil ****s Dutchmen."