I have a hard time believing it is anyone but Katherine’s heart. Although I cannot believe they would start a story line and make a point to show Fredrick is in SB and kill her off, Regina and the EQ made it very clear she gets rid of those who are no longer of use to her. Kat’s point was to keep MM and David apart, and when she realized they were meant to be together, she became of no use to the queen.
My only other suspicion is that it is Kat’s heart but she is not dead (like Graham’s situation). If the queen had taken her under control in FTL it wouldn’t be unheard of that she was locked away somewhere as Belle is. But I find this HIGHLY unlikely
What I would love is for Kat to burst in while MM is on trial for her murder – it would be such a tailspin and would show that someone in the town was framing her. Again, this is HIGHLY unlikely, but it would be fun 😀