No it's not survival instinct, it's plotting: she deliberately used the 5th prince as a shield so that if something went wrong he gets the blame rather than making her own situation worse. She has no idea if there are any long term downsides to using the glutinous rice mortar. The most sensible course of action would be to own up to her lies. Yes she'd have committed the crime of deceiving the Emperor but given how he gave her a chance to own up (by sending the Eunuch) he was obviously willing to be a bit more lenient for the sake of her father so she might be spared a death sentence. She hasn't even considered the impact this could have on her father's career. And before anyone points out she's only 15, children raised in that kind of family would have had the idea that their actions reflect on their family drilled in from a very early age.