Just a few short thoughts, and a little encouragement for any of my readers with connections with Japan. I know that the last month or so has been profoundly difficult.
Japan will endure. In 2100, there will still be a Japan, and it will be recognizably Japanese. If the world as a whole manages to muddle through and maintain its trends in life expectancy, perhaps achieving 'actuarial escape velocity', there may even be a few Japanese folks old enough to have seen this post. They will almost certainly possess the reasonable belief that Japan is still THEIR country.
Your leaders are often venal and corrupt. So are everyone else's with very few exceptions. They're inclined to lie to you to avoid panic and to save face. So too again are almost everyone else's. You might have to take a significant chunk of real estate out of agricultural production for a while. Rebuilding will cost you a ton of money, which will probably hurt your economy substantially. But your elite and your leaders are NOT traitors. They do NOT wish to elect another people. They do not hate you and yours. In this I envy you. Contrast your movie 'The Seven Samurai' and our movie 'Braveheart'. They speak to the radical difference in nature between your elites and ours.
Eventually as your population decreases, your young people will feel that family formation is desirable and affordable in significant numbers. Try to avoid crushing them with debt and thereby suppressing their fertility. But your elites know this already, and, they don't hate you or want to replace you. You trust your elites more than we trust ours, for good reason. You'll probably manage to slog your way through this crisis without doing anything terminally stupid or creating a massive schism between your elites and the ordinary Japanese citizenry.