the only limiting factor is that Earth1 Bitcoins were not created on Earth2....but you would have the same level of security as when every single computer on Earth gets automatic windows software update every day from a THRUST worthy address such as Microsoft.com (instead of NorthKorea.com).
Who says Microsoft are trustworthy? I don't trust them.
What you are describing involves everyone placing their complete trust in a centralized third party. This is exactly the thing bitcoin was designed to combat.
The only security breach that I see is when picking random 20,000 (or more) miners into 1 group is to make sure their addresses (IP addresses) are not fake and instead are all coming from one corrupt source such as North Korea....?
How are you going to "pick" the miners in a decentralized manner? How are you going to prevent other miners from mining in a specific group? What's to stop all the miners from one group moving to another group and 51% attacking them?