Looking to increase you street cred eh?

- How heavy are NJS bikes compared to my previous owned bikes?

Depends on the tubeset. But I believe frame weight is governed to some extent for each size.
Choose 2 only: light, stiff, durable. NJS frames are usually the latter 2.

- Why are so many dents and scratches on NJS bikes? Even on spots I would never guess would have a dent.

Because NJS exporters from Japan buy used/ retired/ crashed frames (i.e. the ones that are roadworthy but not raceworthy failing NJS inspection) that would otherwise be recycled for their steel or sold in a domestic second hand store on the cheap for profit; Japanese culture does not value used things whatsoever. For example, if you're into tuning Japanese cars and the JDM scene, you would know that you can acquire slightly used performance/OEM parts for pennies on the dollar. Gets even better if you're able to buy a container load of stuff.

You're in effect buying a professional keirin racer's beat up ride that they rode the living-**** out of on the daily for training & races. Would you like his NJS socks as well?

- Why the high price? Perhaps because its made in Japan and most or all of them are handbuilt?

Hype. Buyer's perceived rarity. Supply & demand. I recently sold a $150 g shock for close to $500.