For 5.56 its based off of 77 grain bullets. Here is more specifics on the reticle:
http://www.vortexoptics.com/uploads/man_reticle_ar-bdc2-web.pdf
Yes, the mount is designed specifically for AR-pattern rifles.
I would not recommend using this particular scope with night vision goggles. A clip on night vision device would be more suitable to a scope like this.
You should have no problem hitting targets well over 100+ yards clearly with this scope without any additional magnification.
The glass on vortex products is very clear and very well made. The Swampfox glass is also very well. Its a very close comparison, I would recommend the Vortex scope.
You can purchase the mount separately or with the scope as a kit.
You will need MPN: SV-2 found here:
https://www.opticsplanet.com/vortex-switchview-throw-lever-for-viper-pst-and-viper-hs-riflescopes.html
Yes, you can use this scope for a crossbow. I would highly recommend you use a high quality ballistics application and it will provide you with drop values for this scope that will correspond to a crossbow.
The reticle manual can be found here:
https://vortexoptics.com/amfile/file/download/file_id/1149/product_id/3036/
Yes, it should be fine and is covered under Vortex's VIP warranty which is the best warranty in the industry.