Sure, the same way colonies all over the world weren't technically "forced" to adopt customs of their invaders, but did anyway.
I'll start by saying this isn't the real world, and that the British empire actually improved quality of life for some countries (at a great cost, as an Indigenous Australian I know)
But the economic factors which meant that House Hlaalu became more and more powerful weren't a conspiracy by the empire, the fact that the Dunmer way of life was incredibly tribal and archaic is just a fact. Their social order never evolved.
A religion and social structure can remain the same, but not if at the detriment of the countries economy (and indirectly the quality of life for its people), and if Hlaalu had not accepted the empire way the entirety of Morrowind would be bankrupt.
And to your point earlier about the empire pillaging Morrowinds resources, Redoran and Hlaalu own a lot of the mines and sell the ire for a profit, so the empire is trading not looting.
Assimilation would have been necessary with or without a treaty, or Morrowind wouldn't be able to support it's population.