You can take the headrest off in most cars which is what most people do.
Apparently research says there is no difference in incidence of travel sickness between RF and FF but there is a slight difference in onset - it happens faster when RF.
Personally I know it always makes me feel worse when I am RF. I have been lucky not to have travel sick kids so I would never judge for that. You have to put everything into context and certainly the risk of crashing is worth considering, as if you can avoid/reduce the chance of crashing in the first place, that's much more effective than which car seat you use!
It's a bit like BF/FF isn't it? If you take one small aspect into account (nutrition, performance in a crash situation) then it's very easy to see things in black and white and say everyone should BF/RF for as long as possible, but in real life there are always other considerations to be had whether that's medical issues with the child, impact on siblings/others in the family, mental health of the mother, or even the likelihood of a car crash.
Most problems or barriers preventing ERF can be overcome, but not all of them. My only wish is that the info is easily accessible so that people can make an informed choice as you often hear things like "My child was too big for a RF car seat/the seat wouldn't fit in my car/the seats are too expensive" etc, and what that generally means is that the person wasn't aware that you can get compact fitting RF seats, RF seats that accommodate large children or RF seats on a budget (not always all 3 in the same seat sadly), as all of these things exist, but parents aren't always aware of them which I think is a shame.