Is Jupiter really shepherding the main belt, and Neptune the Kuiper belt?
Orbits aren't knife-point balances of speed, distance and direction. For two objects separated by distances much larger than their diameters, almost any combination of the three where the speed is below the local escape velocity results in a stable elliptical orbit whose specific parameters are determined by the combination.
Ceres (and all the other objects in the solar system) are affected in their orbits by the gravitational influence of Jupiter (and everything else, for that matter). However, because the orbital periods of Jupiter and Ceres aren't low-denomination multiples of each other, the influence of Jupiter, while present, remains small, and the orbit remains pretty much the same, year to year.
Where the periods are a low-denomination multiple of each other, however,  the gravitational interactions reinforce repeatedly over billions of years, and can serve to both alter orbits such that the resonance is no longer maintained (see the Kirkwood gaps in the asteroid Belt), or in specific situations, lock an object into maintaining that resonance.
With Pluto and the Plutinos, the fact that their periods are clustered around 3/2 Neptune's orbital period is a direct result of Neptune's gravitational influence over time, rather than in spite of it; The regularity in the amount and the direction of Neptune's perturbations to their solar orbits keeps them near the 3:2 resonance, similar to the way Jupiter keeps the Jovian Trojans near the 1:1 resonance and the Hildas near the 2:3 resonance.
Note that all the orbital parameters of all the bodies in the solar system are changing over time (For example, the perihelion of Jupiter precesses approximately 6.55 arcseconds per year). The Keplerian Orbital Elements of the planets that you may find online are based on measurements taken at a specific time, and define orbits that assume that the planet and the Sun are the only bodies in the universe.