Now, about treaties: Washington Naval Treaty text can be found from here: http://www.bobhenneman.info/Tech Articles/WaTreaty.htm There is a list of ships which RN can retain (Chapter II). There is only 4 battlecruisers (Hood, R&R, Tiger) and 18 battleships on that list (four of them to be scrapped when Nelson and Rodney are comissioned). Now, to get back to my point of making all R's as battlecruisers and surviving WNT. After Jutland, battlecruiser reputation was severely damaged. Therefore I dont believe that even RN wanted to have so many battlecruisers. And we can see that from the list of ships RN was allowed to preserve; only 4 battlecruisers and 18/16 battleships (before/after Nelson and Rodney comissioning). Now, If RN had built 7 or 8 R-class battlecruisers, would USA and Japan agreed that RN could have them all + Hood? And would they agree to scrap/discard those ships they in real life did? I mean, RN has 8 or 9 battlecruisers, IJN has 4 and USN hasn't got single one. Would Japan insist building their Amagi's and USA Lexingtons? Would WNT even succeed then? As far as I know, it took much time and effort to broker that original Treaty.
I'm inclined to think that a slick UK delegation could perhaps swing the treaty this way. Remember that in this scenario they'd be swapping out a BB for a BC and therefore reducing the battline accordingly. In fact, they could probably argue that having the BC's would best serve their strategic (imperial) interests which would be to hunt down convoy raiders / keep the sealanes open rather than go toe to toe in a battleline slugfest I think it boils down to can the persuade the yanks?, if they can then i believe the japs will fall into line.. As for what the others will build in response that's up to them, i believe the treaty limits capital ship construction to xK tons of 35K each, so let them build the Amagis & Lexs. In fact insist on the yanks building the Lex's, cos they were very average and much more suited to carrier conversion, dont convert and you probably set the yanks back 5 years in carrier doctrine..