I would propose the following experiment: build mirror life on board a space capsule that has been sent beyond Earth orbit into a solar orbital trajectory. A large expendable Starship or Orion capsule could have enough interior volume to hold everything needed. Do it remotely by robotic control.

Perform a series of primary safety experiments on board, determining whether mirror life truly does evade immune systems, predation, decomposition, or other biological check mechanisms.

If the safety experiment results are alarming, the spacecraft is not just far past Earth but has a powerful explosive on board and can be detonated in deep space. Any living matter scattered by the explosion would be destroyed by solar and cosmic rays, and it's not coming back to Earth anyway.

A maybe cheaper but slightly less intensively confined solution would be to do it deep underground with a tactical nuke on hand to turn the results into radioactive ash encased in bedrock if the results are alarming.

I think we should do this, since it's obvious that the technology to create mirror life exists. If it is existentially dangerous we should find out under highly isolated conditions. If it's not we can breathe a sigh of relief, and if it is we can take appropriate regulatory action before someone oops-es it.