No you can't use a magnifier behind the prism scope.
No. The Vortex Spitfire HD Gen II 5x Prism Scope uses an MRDS mounting platform for use with the Vortex Viper and Vortex Venom red dot sight. The Holosun 507 series of dots use the Trijicon RMR footprint and would not be compatible.
No. It does not appear that this item has a "zoom in and out" feature.
Recommend reaching out to manufacturer.
This optic will accommodate any Fastfire-footprint optic outright, otherwise, we have a clever adapter that will allow you to mount our Defender-CCW, ST, or XL available for no-charge! Give us a shout if you need one! 1.800.4VORTEX!
This optic was designed to work with the now-discontinued Viper and Venom red-dots, but, we produce adapter kits to allow them to work with Delta-Point Pro, RMSc, and RMR-compatible systems, as well!
The optic can be mounted on a rifle with a fixed post, and overall, it should not be terribly obtrusive. There are several aftermarket risers that could be installed if the user desired a 1.93 - 2.1" mount height, otherwise!
The Vortex Spitfire HD Gen II 5x Prism Scope will work with your M&P-10. However, the BDC reticle was specifically designed for 5.56, not .308. This just means the BDC will not be entirely accurate to your rifle. You will have to use different holdovers from what the BDC displays.
The Vortex Spitfire HD Gen II 5x Prism Scope MRDS Mounting platform will fit either the Vortex Venom or Vortex Viper. The sight that you use is more of a personal preference.
The Vortex Spitfire HD Gen II 5x Prism Scope can be used on an AR or AK. However, the Vortex Spitfire HD Gen II 5x Prism Scope AR-BDC4 reticle is calibrated for use with 5.56x45mm ammo. So the holdovers may not be accurate for the 7.62x39mm ammo in an AK.