Here's a couple of further thoughts.
The Washington Treaty
Now no single act was more important in influencing ship construction for WW2 than this document, designed specifically to avoid the destructive naval racing pre-WW1. The limits imposed by this treaty were not modelled in HoI2 at all. It should be possible in HoI3 to do this. In other words, in HoI2 terms, a breach of treaty limits (simply defined by counting numbers of ships and types - should be a fairly simple AI algorithm) should be accompanied by a penalty, or would not be allowed unless the country had a certain level of belligerence.
The First London Treaty
In 1930 another treaty was agreed. This increased the limits imposed on countries' ability to build through to 1937. No new capital ships, no new carrier conversions, limits on tonnage for destroyers, light (under 6.1" armament) and heavy cruisers. It is this treaty that encouraged the development of cruisers that were armoured as heavy cruisers but armed with 6" guns (like Mogami or the RN's 'Town' class). All major naval poers were signatories to this as well.
The Second London Treaty
In 1936, with the deadline of 1937 approaching, another treaty was arranged. This time only the UK, France and the US took part. The basic upshot from this was that the UK adopted the 14" gun for the KGV class (rather Lion that was meant to have 16" similar to Nelson) and some US treaty cruisers, the St Louis class, but these were actually considerably larger than the rules allowed. There was an 'escalator' clause, that allowed signatories to build ships with heavier guns if the non-signatories carried on regardless. Hence North Carolina could have 16" because the Japanese were already building Musashi.
The Anglo-German Naval agreement
Signed in 1935 this treaty famously agreed that the new Kriegsmarine would be limited to 35% of the tonnage of the RN (except submarines where it was 100%). This overcame the limits of Versailles for the German Navy, and hacked off the French and Italians who were not even consulted.
So how can these be modelled? They need to be as they are critical to Naval design and numbers. Washington is fairly easy (as outlined above) as, indeed in the first London Treaty. Decisions to breach this agreements should be accompanied by a considerable political penalty.
Second London could be an event. Benefits being increased political relations between all signatories, the escalator kicks in if anyone builds anything with guns bigger than 14".
The AGNA of 1935 is more critical. A German breach would undoubtedly have touched a very sensetive nerve for the Brits. The Germans could happily build up to the 35% and then get a warning when further builds would breach the AGNA. A breach of the AGNA would probably have seen increased British belligerence and hostility to the Germans, far more so than continental readjustments, as naval dominance was central to British survival.
How this would fit in with the new HoI3 political setup, well I am simply lost on that one. But given their centrality to what ships you could build and were built/modernised then I cannot see how they can be ignored.
K