Shinju basically waited for a guilty verdict to be handed down, it was intentional? Like throwing a game on purpose...
Let's say Kouichi was right and somebody else committed the murders, then she's covering for someone else?
Here's a crazy theory... what if Kouichi was involved? Being her pro bono defense lawyer he wouldn't be suspected, right? He seems to be in love with her, has noticed her since she was a child, and being in love he would have a motive for getting rid of her 3 potentially ex-boyfriends.