Decide where the sound comes from first. Inventing speech and ambience with a scene is different from synchronizing a portrait to a finished recording. These tasks need different workflows.
No. Speech and articulation can differ; watch and listen to the result.
Specify the soundscape and request only speech and necessary ambience. Switching off all sound is a different setting.
Shorten the line or choose an appropriate duration. Overcrowded speech makes the scene harder to follow.