Hi I managed to free all of the brass fittings on my boat after 10 years corrosion by boiling items in olive oil in and old pan and freeing them while warm. Even managed to save 40 or more brass piston hanks and snap locks
As verdigris is mostly copper sulphate, which is quite soluble in hot water, I would try immersing in hot water [boiling if you like] for a while. I don't have a recipe for keeping bronze bright and shiny, I'm afraid.
Peter.