Huh so my guess ended up being right a few chapters back, but as someone mentioned why would Leo push the MC to investigate if he’s the culprit? It’s possible he wanted to be found out to solidify the idea the princess is dead and leave no room for speculation about whether the body is the princess or not.
The glaring issue about the body’s head being missing is still up in the air, and while the person who did the autopsy “confirmed” it was the princess whose to say she isn’t lying/mistaken? It could imply that by any means necessary Leo wants to ensure that the body is deemed to be that of the princess’s even if means his imprisonment.