Given your departing flight is 8:30 AM, it would be easiest to stay at the airport.
Either
1. Royal Park in Terminal 3, or
2. Villa Fontaine Grand/Premier in the Haneda Airport Garden
Both choices you won't even need to leave the airport - all connected indoors.
You and your mom will land around 3 PM, probably will be 4 PM after immigration and custom. Then you two can check in to the hotel, drop off the luggage, and enjoy the rest of the day in Tokyo. There are trains back to Haneda all the way til midnight.
And if you really don't want to stay at the airport, about your choice of Shinagawa...there isn't really much outside. You would probably have a lot more options if you stay in Kamata or Kawasaki instead.