P -> D -> P -> D is safer if you get rear ended, because lets say you're at a traffic light, you won't roll out into the middle of the intersection and get slammed on the side by a lorry.

Yes, there may be transmission damage if you get hit while in P, but that's what insurance and lawyers are there for. Sue the person who hit you kaw kaw.

That works too, but it's an additional step.

Also, you don't get a new transmission out of it.