Re the Washington Treaty and Furious, C & G.
C&G were not at risk from the WNT. Despite being equipped with 15" guns they were not listed as either capital ships to be retained or scrapped. All parties therefore seem to have accepted that they were not capital ships probably by virtue of their "large light cruiser" description.
Unlike the US and Japan, Britain did not have any incomplete hulls at the time of the WNT. The G3 battlecruisers were ordered just before the Conference but the shipyards had been told not to proceed with construction. They became a negotiating point for Britain.
Back in July 1920, when the RN was looking at its post WW1 carrier plans, it decided it needed two fast carriers. These were to be reconstructions of Furious and either C OR G. Work on the design for reconstruction of Furious was begun immediately and she was stripped down to the main deck at Rosyth in 1921. That was all BEFORE the Washington Conference even began. She was transferred to Devonport for rebuilding between 1922, after the Conference had concluded, until Sept 1925. As she was considered either a carrier, or a carrier in the course of construction, at the commencement of the Conference, the RN considered her "experimental" along with Argus, Eagle and Hermes under the terms of the Treaty. That would have allowed their replacement at any time, regardless of age, during the period that the Treaty remained in force ie up until 31 Dec 1936.
(For the duration of the Washington Treaty Britain (and the USA) was limited to 135,000 tons of carriers. Each carrier was limited to a maximum of 27,000 tons. Under the 1936 London Treaty, there was no overall limit on tonnage but each carrier was limited to 23,000 tons. The RN had argued for 22,000 tons based around its experience with the Ark Royal design process.)
On 27 July 1921 G was chosen for conversion as she could be more easily replaced in the turret drill ship role she was then fulfilling. Then in 1922, after the Washington Conference, the decision was taken to convert BOTH C&G. Initially they were to be copies of Furious but work on a new design was undertaken and the eventual design approved in January 1925. By then work to strip Courageous had already begun and she finally completed in Feb 1928. Work on G also began in 1924 at Rosyth, before she was towed to Devonport, where her reconstruction continued until Feb 1930.
Even in 1926 there was no unanimity in the RN of what an aircraft carrier should look like. By then Argus and Furious were in service with flush flight decks while Eagle and Hermes had islands, with the C&G scheduled to be built with islands. But CinC Med wanted a reversion to flush decks while EinC argued for the island with a funnel as it saved weight which was important in times of limited Treaty tonnages.
The RN had planned a new carrier from 1923/24 but for various reasons kept getting postponed. It began again in 1931 using the experience gained with C&G. At that point the full length flight deck began to appear in the designs considered. That led to Ark Royal, ordered under the 1934 Programme, laid down in 1935 and completed at the end of 1938.
The switch to the Armoured Carrier arose from RN experience and thinking during the Abyssinian Crisis of 1935/36, when the level bomber began to be seen as a major threat to the fleet. The background to their development can be found here.
Action reports and battle damage for the armoured aircraft carriers HMS Illustrious, Formidable, Victorious, Indomitable, Indefatigable and Implacable in World War II, 1940-1942