Honestly, Pearl makes the most sense to me as Pink Diamond's murderer. There's her fanaticism for Rose, her overzealous nature, her occasional crazy outbursts, her reluctance to talk about the subject, and the fact that we don't know whose Pearl she was before the war. On that note, that lawyer Gem specifically called out the absence of any sort of warning from Pink's Pearl, who should have been standing right there. Seems like a decent fit, to me.

Let's take a stab in the dark here. Maybe Pearl was Pink's, and used that position to do the deed. Who better to assassinate a queen than the handmaiden standing right next to her? Maybe Rose helped plan it, maybe not, but the sword in that promo looks awfully familiar. Anyway, I'm sure we'll find out sometime around the next Presidential elections.