Will my iPhone 12 pro stick to a car mounted magnet while in a case?
Yes.
All I'd then be waiting for is for Apple to manufacture a wireless USB dongle to replace my awful wired CarPlay, which is quite flaky, and connects intermittently. My scenario would be
Car dashboard USB port > (in) CarPlay Wireless Dongle > (out) Magnetic charger
That example is with a magsafe case on a magsafe car mount, we don't know yet how the magnet will work with a normal case on a normal magnetic car mount.