My only concern with the movement issue is that a person driving a car without accelerating should only feel gravity. That activity generally doesn't make people sick. Sure there are always minor forces and bumbs, but maybe the ultimate solution is as simple as stimulating the rest of the senses with something? Stick a person in a rumble chair possibly with force feedback and let the eyes do the rest. People keep saying drinking first helps, so I still think it's a matter of getting faster feedback on the display.
You feel a whole lot of things when driving if you pay attention. Acceleration and deceleration are both easily felt as are centrifugal forces anytime there is a change in direction. Those are easily felt in daily commutes, but are massively more noticeable the faster you go. You can also notice this if you watch your passengers. Accelerate with anything more than modest speed and you'll see them pressed back into their seat. Conversely, decelerate quickly and you'll notice their body moved forwards before their muscles react to counteract those forces. Turning will see people brace as well.
Even in a case where you maintain a constant speed, you still have forces, however faint, applied to you as your vehicle is constantly having to apply force to counter the forces causing the vehicle to decelerate. You can see this with a bowl of water.
The easiest way to see this is to place an open container of water in the vehicle. If the container is anywhere near full, it will be virtually impossible to operate the vehicle without the liquid spilling out of the container. That mimics in some ways how your body's vestibular system and cochlea monitor your body's movements in reaction to forces acting upon it so that your body can then react according to maintain balance and spatial awareness.
As to driving without accelerating as you mentioned, there is a solid framework around you that moves with your body. As such for most people, their visual system uses that to align with the vestibular system. And for the most part it works.
But it falls apart for a greater number of passengers if there are frequent changes (braking following by acceleration, followed by braking, followed by acceleration, etc.) that cause a disconnect from one to the other too rapidly for the brain to reconciles. Hence you have many people that can and do suffer motion sickness in vehicles.
Experiments with that in VR are successful to a greater and lesser degree. Hence Eve Valkyrie doesn't cause everyone to feel sick, but there are still a significant number of people that get sick in Eve Valkyrie that don't get sick in a car. Same goes for racing games.
It's likely that in a driving game where you sat in the car and the interior was rendered around you and you only had forward motion with no turns, accelerating or braking that the vast majority of people probably wouldn't have a problem. Once you start accelerating, braking, or turning, however, suddenly it doesn't match what your body and mind are accustomed to. That's likely why even in those game genres there's a percentage of people that get sick where they normally wouldn't get sick in real life.
A rumble chair wouldn't work in the slightest. But a motion chair with 6 degrees of motorized movement (like I linked previously) may be enough. However, those are expensive.
Regards,
SB