I haven't used the battlefield much, so I'm not entirely sure I'm not doing anything wrong, but I got my second squad on the mirror battlefield, and I can't get it home (or have it do anything at all).

Please make sure that you have a correct battlefield and a correct squad selected. If you're sure about these two items, could you please elaborate what exactly is wrong? Buttons disabled, buttons enabled but don't work, an error pops up when you try to do something etc. I just checked your squad on the mirror battlefield using your userdata but failed to reproduce the problem. It was possible to move it, fight with it or leave the battlefield.

My Squad 2 is on the Mirror battlefield, their status is given as "Battlefield" and I can see them occupying a zone there, but all buttons except for the "search a way there" one are disabled. If I click the search button, the "leave" button gets enabled, and when I click that, I get a message that my pets have left the battlefield, but they're still there.

Did you switch your active squad to the one that is on the mirror world now? Active research button makes me think that a different squad is selected.

You're right of course. I'm stupid.

No problem at all! Glad that the problem has resolved :)