Japan was not a military by its choice and legacy of WW2, in India's case if India becomes an economic powerhouse it will also build an effective military force, having a huge economic, military and political power would be hard to ignore. But again, it will take few more decades for that to happen.
this is not true.
your whole argument is based on the assumptions that: 1) india will keep developing, 2) india will be an unified nation forever. both are highly unlikely.
1. with a 30 years economic gap, the current development speed in india is far slower than Chinese in 1980s. yet the inflation is much higher. the government basically failed to regulate the economy. GFC told us this is damn dangerous. e.g. a shitty apartment in Mumbai can be damn expensive, while the slums are killing poors. eventually people will fight for such huge economic gap.
2. historically, there is no nation called india in this world. that is the reason why people in india speak different languages follow different religions. Not saying this it bad, the problem here is statistically, eventually these different ethnic groups want independence. that is also the reason why not all lands are under your central government's control. freedom fighters are actually seeking what want for.