Thanks for your kind comments. I wish there was an undo history too, but I felt that the time would be better spent elsewhere. Game jams, y'know! I tried to keep most of the puzzles short so that it wouldn't be too annoying if you made a mistake. I also haven't locked out any of the puzzles, you're free to skip around them if you get stuck.
You can actually reverse the selection order by holding shift. But even then when there's lots of them it's still cumbersome to find a particular alien. Doesn't really help that the order of selection is essentially whatever order Unity initialises them in. That even changes from load to load in the editor! I think a better solution would have been able to use the mouse to select too, as well as space/tab to cycle through them.
Did you solve all the levels? I added a walkthrough video, just to show that they are all possible...