I know those 3 other princesses are just decoy, but really....how far are you going to protect your precious MC-princess?
Of course he's going to intervene, but very late in the game. I wouldn't count on her saving your prince. I know she's a reincarnated person and not a real child, but a real child, which the emperor should believe she is, would turn out ....who knows how...everything from a psychopath to a unrepairable broken, introverted person is a possibility after this ill-usage.
And that arsehat of emperor actually caring for her? F**k it! He really didn't care with his deeds.