So if LHR decides to let airplanes fly anyway, they'll stop flying the drones?

Or continue flying them in places where it's forbidden, but planes don't go. Which is not 100% without risk and would leave little time to react if they'd leave them (which is why flying there is forbidden), but very far from "intentionally flying a drone into a plane".

In particular, I strongly suspect that Heathrow has zones which, while not directly in the flight path, are still "too close" and will therefore cause them to stop operating airplanes.

As you say, that's not 100% without risk. It's an erosion of the safety margin, and safety margins are there for a reason.

Quite probably. They're not crazy. Not as crazy as LHR would be in this hypothetical situation.