I think it would definitely affect the folding (talking about adding a third more to the depth of the seat), so would require a redesign of the seat. Then, these seats are tested and certified, so would presumably need to be re-certified. Then, 10cm less of the footwell depth (which is going to make it uncomfortable and probably unsafe for the kids). Overall, it really doesn't seem trivial to me.
Asking for the cars to be made so that third row seats could be added later would probably be doable, but I really don't see the law changing within at the earliest 3 years (even if we had a strong case and lobbying started today). Going back, after 3 years or more, and asking to re-register a vehicle is not going to be simple.
I was involved in the expressway permit issue - and even with an obviously out-dated and unworkable law, it still took 2 years to get it changed. At the time, it was also illegal for an EV to run the battery flat and stop on the expressway (but legal for a petrol car) as electricity was not classified as a fuel. 2 years to change that.
Up until the point that we found the legislation says 38x38cm, and the seats are 28cm, I was pushing and working hard for this. So long as the seats were in compliance with the regulations, and it was purely paperwork / bureaucracy, we had a chance to get this done. Now that we know the seats are not in compliance, it is unfortunately over for the short term.