No, I totally understood what you was saying, but maybe I didn't spend enough time explaining it in here.
The fact that you are putting a towel/wedge in to stop a seat moving about is even worse than doing it to stop the seat from getting marked.
If you need the towel or wedge, then it sounds to me like the seat doesn't fit the car as no correctly fitted car seat should have a gap underneath it or be loose in the car.
You have to imagine how these seats move in an impact, first they move towards the dash of the car until they can't move any further be it the isofix tethers that stop it or the seat belt, once it stops in this direction of travel the seat comes back (generally) towards the seat of the car, compressing the seat foam and the seat belt tightens back up as it does this. Now imagine if there is a foreign object, such as a towel/wedge there, firstly you don't know where this could have moved to during the motion of a crash, secondly how are you going to make sure the carseat lands pefectly back on it, and thirdly who checks to see how the thing compresses when the car seat lands on it?
You have to picture that in the testing is the same density of towel/wedge is used on the crash testing, no as no towel or wedge is used, as its not supposed to be there. So who takes accountability for this been there and how it effects the crash test, no-one.
When car seats are designed they are made to fit a generic bench, and also with statisics for the market place of average seat angles and data like this.
Unfortunately they have to be tested on a specific test rig, which is as instructed in the standard, so they can't be tested in a specific car, and could you imagine how much that would cost to do
Its already costly to do the bench testing, never mind killing a car eveytime you tested these seats
I know testing doesn't give a true picture of the real world, but it is the best thing we have and the only thing we have at the moment. It is constantly been revised and reviewed, and this is shown by the introduction of side impact testing soon coming into force for us in Europe,