Seeing as the three know the Princess’s face from seeing the statue they would have been able to tell during the award ceremony if the Princess really was switched out beforehand or not. Though only Leo has seen her face in person before, being her childhood friend, the MC and Kirio only know her face from the statue.
There’s also the issue with the supposed shapeshifting magic that was mentioned. While it would be very difficult to create an exact replica due to the limitations of their imagination for anyone who has not met the Princess prior to this point it would be enough to fool them.
I suspect Leo for various reasons, in that he could have shown the statue to the MC and Kirio to have them focus on that specific imagery of her face and be none the wiser to a possible switch or notice any incongruity if shapeshifting magic was used to copy the face of the statue. In way they would also serve as witnesses to an extent if they unknowingly confirm the Princess was in fact not switched out prior to the explosion.
There are still other questions that need to be answered though, such as If the Princess was switched out whose body is lying on the floor? You wouldn't really decapitate a body unless its to mask the identity of it.