Bug in default environment handling?

Hi, I am still having issues with properly handling the default environment.

1- how do I assign the default setup to an existing V5 file? (Now I import the V5 file to get this done)
2- If I create a new file and go into Environment and then double click the “default” setup then the actual default is changed to this, but I can not go back again, undo doesn’t work and neither have I found a “reset to new file default” option.