only selected new phones are getting redstone builds, remaining including my 520 are getting threshold build, latest being 10586.318. Is there any possibility to get Redstone Upgrade in my phone. I tried changing Phone manufacturer name (from NOKIA to MicrosoftMDG) and hardware variant (eg, for 520, its RM_914) to that of 950 XL (RM-1085-113025) but still cant get the update. Please help me!
No,it is not possible (at least officially).
RedStone builds are available for phones that are eligible for Windows 10 insider preview. I'm sorry to say that your phone model is not in the list of eligible phones for Insider preview. As a matter of fact Lumia 520 is not even qualified for the official Windows 10 upgrade.
Therefore there will be no new builds to your phone.
You can see the list of eligible phones in the below links :