JR Pass reservation on trains

Hi, I would like to find out the following. Please help. When I reached Narita Airport, I understood I need to exchange my EO for the Japan railway pass so that I can start to use the Narita Express to Tokyo station to board my train to Shin-Osaka.
1) Can I immediately do my seats reservation with the staff at the counter in the JR office at Narita Airport for my Narita Express as well as Hikari shinkansen to Shin-Osaka on the same day?
2) How about the returning trip of Hikari shinkansen back Tokyo, can i make the reservation on the same time or I should do it later before I return from Osaka to Tokyo? Planning to stay at Osaka for 3 days by the way.
3) Does the shinkansen door and the station platform level or we need to take another step to board the shinkansen? Because i got an elderly travelling and sitting on wheelchair. Need some advices on how to go about on this.

Appreciate all help here.

1) Can I immediately do my seats reservation with the staff at the counter in the JR office at Narita Airport for my Narita Express as well as Hikari shinkansen to Shin-Osaka on the same day?

Yes.

2) How about the returning trip of Hikari shinkansen back Tokyo, can i make the reservation on the same time or I should do it later before I return from Osaka to Tokyo? Planning to stay at Osaka for 3 days by the way.

I heard that when the ticket office at Narita is very crowded, that they may limit the number of reservations they make for you. Otherwise, it will be possible to also make reservations for your return trip. Reservations for your return trip could also be made at any other ticket office in Japan, including ones in Tokyo and Osaka.

3) Does the shinkansen door and the station platform level or we need to take another step to board the shinkansen?

No, they are very easy to board:
https://www.japan-guide.com/g3/2018_reserved_04.jpg

Thank you so much for your time taken, Uji.