There Some Interest in THIS Project
Hi again Phil, along with the others, watching with bated breath.
With careful use of Phosphoric acid, it will not seriously attack the base metal. When the 'red rust' has all been converted to black, it will have done it's job. 5L of acid is pretty cheap at the local pool shop - at that strength, it isn't as hazardous as it sounds - like it won't burn your skin with with minor splashes like hydrochloric or sulphuric. It washes off/out easily.
My old departed Dad (dinky-di blacksmith) showed me a method of unfreezing (threaded) nuts (and you have a few I see). Hold a hammer on one flat and tap (not hit) the opposite flat of the nut with a smaller hammer. If you have a second set of hands, then a drift + small hammer will work better. Do on as many pairs of flats that can be accessed. As soon as you can get it to move, then your method will work fine. The tapping method will reduce the risk of shearing the stud/bolt. The theory is that the tapping will slightly enlarge the nut.
Good luck, I'll look in tomorrow nite for an update. Daryl