The two species are in a symbiotic relationship, i.e. the Menk do not perceive the Valakans as evil.
HOSHI: The Valakians don't let them live where the land is fertile.
HOSHI: The Valakians give them whatever they need. Food, clothing, medicine.
HOSHI: He says the Valakians are good to them. They protect them.
So your civil rights movement idea is nonsensical and a result of our human perception of the Menk as exploited. Which is the core of the issue, forcing human values down other species' throats while believing that you are doing something ethical. The road to hell is paved with good intentions.
I already mentioned what I guess to happen when the Valakians slowly die, the Menk would probably start to rely more and more on their own.
If the Valakian assistance is cut off relatively sharply the transition period might very well be nasty and imply a lot of suffering but sooner or later they will learn to grow food, build a new society, prosper and culturally evolve unlike in this relatively static relationship with the Valakians.