[crotchfire]

that the "driver" (libinput) should not send "simulated" events

As long as they are marked as being synthetic, why not?

If you mark them with a "this is a synthetic event" bit, the compositor/windowmanager has all the information it needs to decide whether or not to ignore the event (because, for example, the pointer moved out of one window and into another).

The driver generates the synthetic events, the compositor/windowmanager decides whether or not to drop them. Done.