In my opinion There was nothing wrong in slapping him just because she slapped her husband doesn't mean she's lost all sense of duty towards him or thinks it's okay to do so she is simply fighting for her self respect which every woman has a right to do. It's an in the moment thing and rightly so. If the queen herself is too weak a woman then what example is she for her people.
Now ..
The "Aap ko agar yehi chahyey tha toh hame keh deti yey kaam ham kar de Te " was a bold statement from Rana !
So that means he would have put his dead wife aside for gayatri so she wouldn't have gone else where is that what he was implying let's not joke we all know that would not have happened but that ego yesterday was not false maybe there is another side to Rana that we are just getting to see. One that will do anything to keep his ego in tact!