No, they won't need a new ECU because they are already CCS cars. They'll need a dumb adapter if they can't get a new inlet installed.
Also, whatever water cooling magic keeps the charger plug and its terminals cool won’t cool the adapter.
Even the little charge-a-J1772-car-off-a-Tesla-level-2-charger hack adapters are generally dumb adapters designed for 40A or so and are not safe if connected to a full-spec 80A EVSE.
Or is the issue trusting people to buy the right one?
No, that's done by the car. The adapter just changes the shape of the plug.
The car controls the charge curve.
Exactly. If a car uses an 800V battery and can charge at 500A, and the charger can supply 500A at 800V, and the adapter is only rated for 150A, something needs to tell the car not to draw more than 150A. Similarly, an adapter rated at 400V that is used with an 800V car needs to tell the car or charger not to charge at 800V (which may mean not charging at all).
If your car can pull 500 amps then you should get an adapter rated for 500 amps. Here's an example:
In the picture, you see a dead charger adapter converted into a new Tesla standard one. It's a major pain for other brand EV owners
In 1 year, almost at every charging station, you will have to deal with plugging converters in snow, rain, etc, making sure communication works.