A great point, but not very controversial. Mainstream historians often propose that the USA's late 20th century economic, technical, and military prowess was a direct result of WWII.
As far as Japan goes - IMO their population is too old to solve the problem on their own. No matter how much propaganda you feed them, 50 year old women cannot have 4 kids.
Japan isn't completely devoid of fertile women. If you convinced them all to want six kids the population would rebound pretty impressively. Hell, my great grandmother had twelve children, and it wasn't that uncommon at the time.
Personally I think the Japanese population will rebound without any meddling. Every large population has sub-populations of people who have lots of kids. Think Mormons in the US. Eventually one or more of these sub-populations will have enough critical mass to affect the overall fertility rate.
In terms of militarization, well, the Japanese navy is still the most powerful navy in Asia. It would make pretty short work of the Chinese navy in any conflict out of range of Chinese land-based aircraft. Just calling a navy a "naval self defense force" doesn't mean it can't hold its own in a fight. This will still be true even when the Shi Lang is fully operational, though I agree the trends are in China's favor.
As far as Japan goes - IMO their population is too old to solve the problem on their own. No matter how much propaganda you feed them, 50 year old women cannot have 4 kids.