OK, so this is what I would "expect" has to happen. The backrest needs to come out and then, possibly, access holes would need to be cut - but they'll be hidden by the cushion once it's back on. To get the cushion off, look under the bottom side of the cushion for a few screws. Once those are out, the cushion should slide upwards and off. It's possible there are no securing screws.
However, you might get lucky with the right extensions on a socket and be able to reach the nuts on the grabrail by going through the storage locker under the seat. There may or may not be enough room - but you can check.
Most likely there's rusting because of the lack of air to that small space between the grabrail and the deck. Use a little butyl rubber tape or sealant when reinstalling and that should take care of it in the future.