Then why didn't Imperial Japan fall when Meiji took on Western advisers, adopted Western clothing, set up a Western-style parliament and constitutional monarchy, abolished the rule of the Daimyo, etc.? These seem like far more radical changes than switching religions, given that the Meiji Constitution guaranteed conditional freedom of worship, and this is before the cultivation of State Shinto. I don't think it's really fair to call Christianity the barbarian religion at this point either, given that it had been in Japan for hundreds of years at this point, and there were local Japanese Christian movements, both Catholic and Protestant. Even if the emperor converted to Christianity, that still does not mean that he would necessarily discourage devotions to him, either. A certain degree of syncretism is to be expected- in OTL South Korea, for example, even Christians visit shamans, and non-religious Japanese still visit temples. I don't see why Shinto rituals couldn't be secularized, which to many Japanese they practically are.
What if it's a few years after the war, perhaps even decades?