IIRC, Churchill's sacking came about because of the Dardanelles debacle. He was the scapegoat there, the real fault lie with the commanders on the scene, but that's another discussion.
As for Coventry, I recently read that the story of Churchill knowing Coventry was a target but deliberately didn't warn the city is a myth. All the British knew in advance was that there would be a raid that day, but they didn't know the target. Can't recall where I read that, unfortunately.
You are correct on both points. The myth that Churchill sacrificed Coventry to keep the Ultra decryption secret was (I think) first brought up in the book Intrepid. The British historian and politician Nigel West (real name Rupert Allason) has debunked the story in one of his books. (I think it was Counterfeit Spies, but like you I am not quite sure.)