Legally, I believe we can consider them a threat until they leave the house, so long as they disengage any attack posture. If they persist with the confrontation but are merely driven outside, then the threat remains imminent, and justifiable defense is warranted. Doubling back may only lead them down another path of doom. If they don't get the message on the first volley, they surely will the second one! Give me a chance to reload/rearm, and I will likely get something bigger and more powerful, as well as take up a more advantageous position from which to deliver.

It's been my experience that the police are not there to protect and defend, unless you happen to live next door to a police station. Usually they are there to secure the crime scene and help clean up the mess after the fact. I have no problem with relaxing my attack posture when properly identified officers show up at my home in response to being summoned.