Also, I was thinking...
I think Krethusa might be a huge idiot. She doesn't trust Morathi's Khaine schtick, fair enough, but we're past the "queen posing as an oracle" phase. Morathi is a literal god now. The only reason Krethusa has any power is because Morathi freed a sliver of divinity from Slaanesh. So she's just mad that Morai-Heg isn't around anymore? I think the time for rebellion was before Morathi deified herself.