I flew MAN to JNB last year . I don't need an aisle chair but did book a wheelchair and assistance onto the plane.
Etihad organised the w/c from check in and they put my own w/c in the hold. The w/c pushers took me through security and to the plane door. I am able to walk to my seat. They should have an aisle chair on board.
I had a stop over and there was a w/c and assistant waiting to help me and got me through security again then came back for me in time to board the second flight.
FA's were helpful and put my bag in the OH and even found me a seat with more leg room. Turned out to be a bulkhead seat with babies next to me but I love babies so I just willingly helped the mum out.
Don't worry, I am sure they will look after you at the airport and on board and I hope you have a good journey.