Yes, the Sniper AR .308 Quad Rail will be compatible with M&P-10 rifles that utilize a 2-piece handguard and a mid-length gas system such as model Sku:11532.
Not sure. It should fit the gun, maybe you need to update your gas block.
No, I don't think so.
This one is 12.5", don't have 15" or 16.5".
Yes, it is. The end cap should be round end cap, not triangle end cap.