[Suicide Junkie]

Reducing spin DOES NOT decrease the gravity by any measureable amount.
The rate at which you add mass means absolutely nothing.

The spin-rockets would have to be landed on the surface carefully, which means matching the spin rate of the surface so as not to plow into the asteroid and leave a crater full of expensive shrapnel. Installing the rockets would not change the spin rate of Ceres, and would increase its mass and angular momentum by an unnoticable fraction.

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Unfortunately, searching for '"Ning Li" site:ua.edu' turns up zero items.
Ning and Li appear separately, though, as different people.

And if you open it to non - University of Alabama sites relating to spin and gravity, you get a buttload of crackpots, alien/UFO sites and such.

I'm sorry, but Newton's laws are the way it is, to great precision on the scales we are talking about.

Again, this is simple grade 9 physics.
Learn the basics, please.