As for the aliens motivations, I'd suggest a genuine desire to help humans. This actually strikes me as one of the most realistic reasons for an alien invasion. Invading Earth for resources or living space is pretty silly unless you invoke handwavium or make the aliens morons, but I can easily imagine a technologically advanced alien civilization taking one look at Earth and concluding that these people could really use their help. Many thousands of humans die each day, mostly of things they could easily prevent (like cancer, AIDS, and aging). A large percentage of Earth's population lives in poverty. The planet is plagued by war and violence. There are all kinds of retardo religious groups going around causing all kinds of preventable suffering for their sake of their retarded bigoted Dark Age beliefs. It's pretty easy to imagine a civilization that has eliminated these things taking one look at us and deciding they could make this place better off by taking over it and imposing civilized order and bringing their superior technology. Meanwhile for a civilization with even borderline effective postscarcity enabling technology much of the potential blood and treasure cost of war is removed or made trivial.
Of course, in the tradition of paternalistic do-gooders the aliens underestimate the difficulty of doing this. They underestimate the degree to which humans value their independence and resent outsiders coming in and telling them what to do "for your own good". They underestimate the complexity of many of the problems that plague Earth, and their simplistic attempts to solve our problems often backfire with unintended consequences. They underestimate the degree to which people often cherish ideologies, traditions, and social structures that hurt them. They underestimate the difficulties that will be created by the fundamental differences between their world-view and ours. And because of this they handle things badly, especially at the beginning. They underestimate the resources they will require. 100,000 troops should be enough to control
Just imagine there's some protest in Time Square or something. Some idiot throws a big blunt object at a combat robot. Their fast but naive AI brains take .017 seconds to decide they're under attack and begin returning fire. With their superhuman senses and reflexes every shot is a kill, and they can fire 200 times a minute. In the time it takes the horrified supervising officer to realize what's going on and order them to cease fire they've methodically mowed down the entire crowd.
I think that would be really cool. The aliens have fundamentally noble motives, but the whole thing turns into a clusterfuck that actually makes the situation on Earth worse instead of better. Grey and grey morality!
Also, make it hard SF. It would really work with a hard SF set-up. It takes a lot of energy to get anything to another star system so the aliens have a big incentive to scrimp on the size of their invasion fleet. And you'd have a sense of a handful of us vs a hostile alien world with the nearest help decades away developing among the aliens, encouraging them to become more aggressive and bloodthirsty over time as they increasingly stop worrying about helping the humans and start worrying about keeping themselves from being wiped out by the resentful horde of angry savages.
This could be very cool.