At your destination you have plenty of rock/sand/dust to construct shielding, plus the instantaneous 50% reduction in cosmic radiation thanks to the planetary mass on landing. In transit that's not an option, it would raise the mass constraints far too much for current propulsion systems. And we're a long way away from effective magnetic shielding.
Orbital insertion requires that you be going slow enough to pull it off. Ceres low mass means you orbit it at very low speed, so you have to approach at very nearly its own orbital velocity. And you only have maybe a month to get from Earth to your destination before the travelers suffer permanent radiation damage, according to all the various Mars plans I've heard. You can do nice orbital insertions f transit time isn't an issue, with humans on board you're in too much of a hurry and have to slow down at the far end. Ceres is roughly 2.3x further than Mars at closest approach, so you need 2.3x the average speed to reach it in the same time window. Which translates to 2.3^2 = 5.3 times the kinetic energy (K=1/2mv^2). Its orbital velocity is also substantially slower than Mars, so you have to slow down further (18km/s versus 24)
If the centrifuge doesn't spin in the same plane as the surface then for the bottom half of your rotation real gravity will be pulling you "down", while for the top half it will be pulling you "up". Net effect "gravity" will the pulsing or strobing at your rotational frequency. Though at only 0.03g you might not notice the difference. What I described is pretty much the "spinning walls cylinder" or "spinning suspended chairs" carnival ride, rather than a ferris wheel spinning fast enough that your seat always points away from the axis.
I didn't say spin-stabilized concentrators would be good for Mars. Mars gets ~3x the sunlight to begin with. Freefall habitats get practically free spin-stabilized solar concentrators. Ceres gets neither.
The ocean doesn't wear sand into spheres, no. At least not as fast as new ragged sand is produced (though those tropical paradises with long stable beaches do tend to have much smoother sand than comparatively rocky ones) . But it does wear down the sharp edges it had initially. And I don't care about thrown around dust on Mars - the rovers have proven it's a non-issue. Clinging dust though is going to work its way into every seal and gasket you have, and the sharper it is the faster it will destroy them.