Kagehikokiri-- I rarely does spa while I am in Japan and most of the time I did it while I am in a ryokan. In terms of pampered level the mo is better but treatment wise , for me Aman is better but for my wife mo is cleat better so it's really depend on which therapist you get. To each of his own, I prefer traditional shiatsu or Thai massage where few hotel does really well whilst most luxury hotel focus more on facial treatment , body exfoliation, aromatherapy....etc
I am not sure if you referring the ground floor as hotel lobby versus the otemachi ground floor as the hotel ground floor has nothing aside the evator to get to the hotel lobby
All hotel floors is open according to yomiko but they limit how many room is occupied at the moment they will put guest to their prefer room and floor .
No bill to sign at restaurant but instead of other Aman where they recognize who you are here they ask for your room number and you have to check in at the spa reception facility every time you use any of the facility and you also have to sign the waiver which I found annoying