One hotel I know is Hotel Quintessa Osaka Bay. https://quintessahotels.com/osaka-bay/

You cannot take one train to Yumeshima. They have a bus running between the hotel and the West Gate of the Expo (some 15 to 20 min.) You need to book seats. Probably wheelchair accessible but need to check with the hotel. The hotel has one accessible twin room.

https://tokyo-haneda.com/service/facilities/coin_locker.html

You can use Baggage Storage Services at Terminal 3 of Haneda Airport.
https://tokyo-haneda.com/en/service/facilities/baggage_checkroom.html

It would be possible for them to keep your luggage more than 14 days by presenting your departing flight ticket.

Possibly @ohayotravel may know

Discount tickets are sold only to those with disability certificates issued under Japanese laws. So I think it is risky to purchase this ticket. For each exhibit, the staff may be flexible and you might be treated same as the people with Japanese disability documents, but it would be situational and judgemental. Sorry it may not be so helpful..

https://www.tic.or.jp/home/foreigner/

This hospital is able to do medical check with English.

But I don’t know details. So could you ask them?

Thank you, Pao! I appreciate your quick response and help. I’ll check it out. Have a great day!