It looks like the side pieces are made from aluminium. Could you use heat to try to move them- aluminium expands more than steel- but the chances are that the electrolytic action between the two different metals has fused them together.
If heat doesn’t work I would try the brick acid soak for a few days. It should eat into the corrosion and hopefully free up the cotters in the frames with an added bonus that it will de-rust the steel parts as well. The only trouble is that I haven’t used acid on aluminium, so you may want to monitor it closely (and put it outside because it gives off Hydrogen Sulphide- bad eggs- which can be harmful). Make sure that you soak the parts in a dilute solution of bicarbonate of soda afterwards to neutralise the acid as well.
Don’t blame me if it all ends in tears!