I’m applying penetrating oil each morning and evening since discovering the nut was rusted on, and tapping the sides of the nut to set up vibrations that may microscopically open the gap between threads of the bolt and the nut so the oil can spread further through the threads. It’s a large nut (25mm diameter, 2.50mm pitch) so this could constructively go on for several more days. Then I place the wrench on the nut and try it, with a pulsating push, and in each direction. The clapper is tied up to keep the staple bolt from turning.
The next stage, if tapping is not enough, is to carefully heat the nut with a torch (lower photo) to expand it relative to the bolt.
Once the lower nut can be turned, its threads should be cleaned like the upper nut’s were, but it’s not clear how this could be done since the substantial weight of the clapper is supported only by the nut. Ideally this weight would be supported while the nut was run up and down and the threads cleaned. Then a dab of lithium grease (middle photo) would go on the threads to prevent further rusting and ensure the nuts don’t seize up again.