I don't think Liesa was at any point implied to be evil. Not only does her commander bio confirm the benevolence of her motives but even back in SOI it was clear Avacyn was always iffy and in this case possibly in the wrong.
Everything's fair game but I hope not. The archangels are special because they were born from essentially Innistrad's big bang, no ordinary angel should be in the same role as such beings and perhaps tellingly there's not a single non-legendary multicoloured angel to be found even as the flights are clearly multicoloured.
My guess is that it has something to do with the Celestus, since that ***** fixes the balance between day and night. The ritual to bring back the day will probably fail based on Olivia's ambush, but it will generate enough light energy to ressurect Liesa.