Having a poly shielding means you don't have anything underneath your caps and so it does not pick up any noise. It also means you couple lesser noise from your substrate and you also inject lesser noise. Always recommended unless you don't have the area.
1. I don't have access to your particular PDK and I cannot say anything about it.
2. Yes. It's a common way to increase the density of decoupling capacitors. Stack MOM on top of device capacitors (make sure they are connected in parallel and not series).