It depends on the ship and the ports if you tender or not.
You will tender in Hilo, Big Island like us on the Radiance. But we will also tender in Maui. But NCL uses another port and thats a docking port.
There are so many handicapped people on board every cruise ship. They are prepared to help.
I´m not sure about some help during shore excursions. I´m afraid, that no tour guides will help you pushing you.
I saw many power chairs, but on RCI ships and there were different types. Many handicapped people using the normal wheelchair and the power-chair, depending on the situation and the place they wanna go, like the dining room.
Hope somebody else can answer your questions better than me.
Have a wonderful time in Hawaii.