Rock I know you did, and as a guide it's fine, but as fact I immediately saw things I can find links to to prove are incorrect, for example the second hand carseat thing, that was all I was saying. But of course your example is fine - presumably your family would tell you if they had been in an accident...
FWIW, I know someone whose relative died skiing. I still ski, and I allow my children from age 2 to do the same. I know someone whose relative died from a tractor related injury and yet I'll allow my BIL to scoop up my children in his tractor cab as I personally think they are massively safer inside a tractor cab than on a farmyard with a tractor reversing. I read that the owner and inventor of the segways died on his segway, I'm allowing my oldest child to have a segway party. People die doing horse riding. I allow my child to canter on a pony and even (gasp) do jumping. I know people die crossing the road, riding their bikes, etc. I know there are safety measures in place (for example, hard hats, seatlbelts, safety teaching, guidelines) and it's INCREDIBLY easy to say do everything by the book, make no unnecessary risks, take all precautions. But sometimes, you have to make an educated risk assessment of all the dangers.
FWIW, the reason why my own daughter occasionally travels in the front is because she undoes her seatbelt. If she's next to me, I can prevent that happening with a glare and a distraction. We have had two years of preventing her from ondoing her seatbelt - every single safety device known to man has been ordered and tried, as well as several expensive carseats. By being in the front, with the seat as far back as it can go, yes, she is some more danger than the back but SO MUCH more safer in the front with her belt done up and correct than in the back undoing it for fun and giggling when I am doing 70 down the motorway.
I would never do it if it were illegal, and choose not to do it when I have a choice (basically somebody else in the car to help distract) but it is not illegal and I still stand my my early answer that the OP is being very unreasonable in wanting her ex to conform by her standards.