Both ANA lounges have showers and they are fine.^
Plus you can chill with some noodles and sake after and really unwind.
Not sure about getting 2 guests in, so I'll let someone else tackle that.
FWIW, the last 2 times I tried to use the NRT ANA *G lounge while flying on a UA boarding pass, the attendants reminded me that there was a RCC for UA flyers.
I thanked them and said I prefer the ANA lounge, so they bowed and said OK and let me in.