I thought there was going to be a final twist where they realised that Princess Victie was secretly Kirio the whole time, and Leo's whole plan of replacing the saint's face with his own and preparing a corpse with a destroyed head in advance was all so nobody would be able to recognise Kirio and she could live freely as she wanted. I totally thought we'd been building up to that the whole time.
There was a bunch of strange behaviour between Kirio and Leo throughout the story that was never really explained, but I thought the dead giveaway was supposed to be that, in the dinner scene where everyone's having a meal with the royal family, which is the only time we ever see Kirio and the princess in the same room, Leo is nowhere to be seen. We only see him in the next scene with the explosion, which takes place several hours later.
If Leo conspired with Kirio, the doctor, and the maid, it would be simple to sneak a headless corpse into the the sealed chapel, and falsely identify it as the princess. Not to mention, the idea of Leo crushing and eating the princess' head for no reason other than to make the case slightly more dramatic, is just insane. She was his childhood friend; if he cared enough about her to kill her to free her from her miserable life as a puppet saint, he would definitely care enough about her to frame himself as some mad cannibal murderer so she could live a happy life under another name with no risk that anyone would recognise her.
Your suspitions bout Kirio might lead to something!
This isn't properly explained in the manga, but there are hints that suggest Kirio might be the mastermind that made the whole incident possible, just to reach her happy ending: dream job, engagement with Van, noble status.
She purposefully disregarded the freeze burn marks,
had a knife on her for no apparent reason,
her feelings for Van and sudden unnatural interest for the statue were the excuse that allowed Leo to show Van the statue,
her grades suddenly dropped making Bob surpass her, leading to her being the last one in the ceremony (giving Leo an alibi for a longer time)
she gave Van the last hint allowing him to solve the case
and many more instances of strange behaviour.
Van thinks about this in the novel and is kinda scared of her, but decides to dismiss this intuition for the sake of his happiness, I wonder if this will eventually come to bite him back XD