I wonder if she was buried alive by all these people who claim they wanted to protect her and the emperor was a former lover who couldn't stop it from happening. Because of this he's pissed off at the people surrounding her. So the one on her side is actually the opposite of what's on the surface.
How is she stupid? She's got to be disoriented and doesn't have information and biases the audience does but still manages to deduce that she's a figure similar to the pope, that the emperor has some fondness for her, she's important to people around her, and put forth a major bluff at the potential cost of her life. From the looks of it it's been less than 3min since she got up from that coffin. In that time, she's had to come to terms with dying, not being in her original body as she expected, realizing she was isekai-d not reincarnated, being in a foreign world not a european country in her home world, speaking a different language, getting up from a coffin, magic existing, an empire existing, a message from god, an emperor physically assaulting her, a violent confrontation between the man in front of her and people surrounding her, the people around her dying, an irate violent man, puzzling out roles and intentions, and her sibling appearing. We know she's safe and important because she's the MC and he's clearly the ML. Sure it's likely she's important, but there's still say a 30% chance maybe that the holiness title isn't taken too seriously by the emperor even if the people around her think it's important, or maybe she's a servant of god like a monk, so disposable. From history, most saints get sainted AFTER they're murdered by kings and queens for upholding a saintly ideal. I wouldn't want to gamble on the hope of a 70% chance. It could literally be game over right there. If you're rational, value your life, and are treating this world as a real world not some game like protag-hoe antagonists in otome isekais, you don't assume you're an invincible special person upon rebirth. Some people think they're god and can do so much better in that situation.