From
this wiki article, it seems like the Naval Treaties (and most events after the Meiji restoration) weren't causes for the rise of militant nationalism, but either symptomatic (in the cases of the coups and the military's adventurism) or catalysts.
The Japanese, AIUI, were influenced pretty heavily by the Prussian system and this combined with the fact that their elite were so overwhelmingly directly descended or former members of the land-holding military elite that had dominated society since (at least) the 1600s laid pretty fertile ground for the crazy train to come along.
It looks increasingly hard to justify a non-insane Japan without fundamentally altering its societal structure. Maybe a PoD where the Prussian influence is mitigated in favour of British, but the problem persists that Prussian militarism is so damned
attractive for the elite in Japan that I don't see how it can be avoided.
Just from reading up on the May 15th incident, Japanese society was beyond crazy at that point, and I think the roots are deeper than anything of the 20th century. 350,000 signatures (in blood

) on a petition to acquit the officers who
murdered the Prime Minister, 11 severed fingers of people who wanted to be executed in their stead. These are not things you see in a stable society...