RSBMThanks PD and everyone else for replying, I try to answer all and I also have photos from yesterday at Cott beach to post later. I am still not sure how important knowing how the car got into the ocean is, but I think it's probably important to try to establish the facts if possible.
Everyone seems agreed that the car was found Wednesday morning on 22nd June 1988. It was seen in the ocean with its lights on and upside down in the middle (approx) of Cott beach. It was towed to shore. There is a picture of it after it was retrieved (and maybe another image that I still haven't sourced after retrieval) on its wheels with the roof squashed to around the bottom of the window. Both back doors were locked. Both front doors were unlocked, windows were all up except for drivers, which was down. Lights were on and it was estimated by this and battery, that car had been in water for a few hours before it was found. Back seat had been washed up south of Cott groyne on Monday 20th June in afternoon (this was from a newspaper report in the days after JC went missing. All later reports just say the back seat was found washed up of a beach south of the groyne, without saying when).
Car was on it's wheels on sand, it weight over a ton, 1,150 kgs, and would have been very difficult to put on its wheels if it was towed along the sand upside down. I'm assuming the car was righted in the water, when it was a lot lighter. But if it was dragged along the sand, I think you are correct and it would have done damage, I could try to calculate how much. I'm assuming it would have been towed up the sand on its wheels where it ended up as this seems logical but may not have been.
Pillars were still attached to the roof and undamaged after being hit by a train. My assumption from this, is that pillars were strong.
Back seat? Back doors locked, windows closed. My memory of the event favours the reports that it was washed up on the same day of disappearance, later reports that I've read don't seem to mention when it was found, and if true only makes the scenario more unbelievable.
To me having the back doors locked would be common in a car of that era as I suspect central locking came later. Having the front doors open would be a normal state for 2 people in a car. If the front driver's door didn't open from the outside it probably wouldn't matter if it was open or not.
I can't see that it would be possible for the backseat to float out the window.
I also think that this sounds like a job for more than 1 person. It seems like a lot of things that could go wrong with this.
For what it's worth while I once thought JC was probably a victim of the CSK, relooking at reported evidence my gut feeling is they are not related.