1 & 3) Variable Gauge & Dual guage are two entirely different things. You explicitly mentioned about "dual purpose dual guage tracks". That Bullet Train in video is able to run on either standard gauge or meter gauge by switching between them, not on "dual gauge" track. Talgo is also able to "switch" the gauges, and run on either one of the gauges. There is no high speed railway system in the world which uses double gauge tracks. They run on a single gauge track only (which can be of any width).
And secondly, China & Japan are developed countries with much higher GDP. They are world leaders in Railway techs & innovative, and meanwhile India is a developing nation which is...
more... only good in importing stuff. We can't afford to establish such expensive and complex infrastructure which is expensive to construct and maintain.
2) Just tell me whether that track is suitable for 200+ kmph operations or not? You have quoted yourself that those tracks are in industrial and goods sidings.