Take a deep breath. Child seats, as far as I'm aware, are designed so that children can open them to escape if needed.
Some cheap car seats can perform well in crash conditions, some expensive ones do not. Price is not a reflection of safety. You may have to do your own research. I rely on Which? testing, but this is not necessarily going to give results consistent with other tests - the whole thing is wildly confusing.
In a head-on collision, a rear facing seat is far safer than a front facing seat as the force exerted on your child, particularly the neck, is vastly less. However I went for a forward facing seat (extremely car sick child) with a body shield instead of a 5-point harness, which spreads the forces in an accident better than a 5-point harness system. Body shields may not be effective if the car rolls at high speed, I gambled on the fact that I wasn't that likely to be in an accident, let alone rolling my car. Also, unco-operative children can slip out from the body shield if they want to, just as they can open a 5-point harness if they want to.
The whole thing is vastly confusing and you'll need time to consider what's best for you.
I don't understand how seat fix relates to car sickness?