I don't think the reflective barrier under you will provide any meaningful radiant benefit. Radiant barriers provide the most benefit when there's a big differential between where the heat is radiating from and to.
The sun to your body has a 6,000C temperature differential- reflective barrier works great as a sunshade.
Your body to a clear night sky (absolute zero?) has a 300 C temperature differential- Reflective barrier could work well.
Your body to snow has 37 C temperature differential- Reflective barrier not so effective. Also consider that the mass of your pad will block radiant energy loss as well. Radiant barriers really need to be suspended, not in contact with the two masses that you're trying to block heat transfer between.
The thin mylar space blanket is cheap, light, and fairly tough for the weight so it may be good for a bivy bottom as long as you are willing to replace it when it wears out. Just don't expect miracles in heat retention.