After rethreading my Raleigh Sport, I needed a magnifying glass to tell the threads weren't cut into new metal. 20mm is one heck of a lot of engagement, I wouldn't worry about steel threads being a bit compromised. After all the cartridges work in aluminum, and the left side threads are plastic.

On my Sport, I faced 1mm off each side of a 70mm shell to use a 68mm BB. The shell on my Twenty measures 77mm so I'd need to face 2mm off each side and use a 73mm BB. Shouldn't be a big deal as the shells are thin and probably just 1020 steel. 15mm of original threads, minus 2 for shortening means you would have 7mm in new, steel threads and you should get at least 50% strength out of the old ones. Sounds like "Plan A" to me.