The idea of 'add it later' armour is an interesting concept, and one the IJN sort of did with Hiei during her conversion to a training ship, where her armour and guns were reduced and added back on later. In this AU both Kongo and Hiei have this done, Hiei as a training ship and Kongo as a target.
So you can use this trick for treaty ships to increase their power after the abandoning of the treatys. The Mogamis and the whole shaddow ships program were constructed with a philosophy that might well include such ideas.
In Germany, a byword is "Where is a will, there is a way". If Japan really wants to cheat, it can cheat. On a long rear of a BB, an empty barbette my be fittet and just planks put over it. This saves weight and looks like an ordinary ship with just 4 turretes, in case of war the planks are removed and an already combat ready turret is added. THings like this were never done, but is this really a reason to declare it impossible? By doing so, you can weight and still have much stronger ships after a short time in the shipyards.
To the "naval point conversions": The required shipyard is several times above 10, especially when ships are converted to a totally differend kind of ship like the CHitoses. In this case, it is often 25. I hope and haven't tested so far that this means the ship is added to the building list and the conversion costs 25. This has to be tested though.
Without any doubt: I am the spawn of evil - and the Bavarian Beer Monster (BBM)!
There's only one bad word and that's taxes. If any other word is good enough for sailors; it's good enough for you. - Ron Swanson