One of the things you do with ADR to make it sound more like production sound is to pitch it up. In the excitement of the shooting set the actor tends to talk louder and higher. In an ADR session, the director typically has to push the actor to get them anywhere near the level of vocal performance that came from the set. If the recording of ADR were treated more like shooting the movie it would almost certainly be better. Ambience is widely used as a synonym for ambient sound.
Ambience usually means the background sound accompanying a scene.
He dropped the previous room tone and shifted the ambient sound. He also changed the spatial placement of the ambient sound - from left, right, and center speakers to surround speakers. Ambience helps establish the scene and works editorially to support the picture editing by, for example, staying constant across a picture cut to indicate to the audience that no change of space has occurred, but rather only a simple picture edit. Conversely, if ambience changes abruptly at a picture cut, an indication is made to listener that the scene also has changed.