Brandt should be an obvious suspect, since his flag revealed he was actually the nefarious pirate king, and samsons head was rolled up in it. Amara obviously has motive too. The coulbournes aren't shy about their distaste for the royal line and grey did insult the king so they could be suspect too, though that's a bit more of a stretch. But I mean, people are gonna be frantic and desperate for answers and retribution. I think whichever PCs aren't accounted for (whoever dropped from the rp) can be held on trial like maybe evidence was planted or something. Like some can be killed in the mob frenzy and others can be executed for assassination or used as pawns or something.
Also for the current issue of succession i think the queen and the princess can have a cercei/tommen thing going on, or like the queen has full power until Madeline is of age.