So if ghosts are apparently valid testimony, why didn't Maya channel Mia to clear her name. Did Mia just not know who killed her? Why not do that for all of the murders?
You mean in Turnabout Sisters?
Because Maya wasn't able to use her powers unless the circumstances were super dire, and even then it was dodgy. By the time she's actually able to channel Mia, they've already accused Redd White
The general excuse for this is that Maya's untrained and channeling ghosts can have mixed results. Spirit of Justice
justifies them not doing this in different ways