To sand the ABS paste, start with a low grit and work your way up, even wet sanding when you get to fine grit. Then give it some Novus to get the shine back in!
Can you clarify what the issue is with the knee ammo belt? It's positioned up over the ridge in the front, as it should. Yeah, looks a bit short on the sides, but it's not a deal breaker. You're just looking for a solution to get a new knee ammo belt that is longer at the sides?