@comeonnow0 oh my bad i thought you asked why the bride's father isn't there and why the Emperor's the one blessing their marriage.
about the whole being tired from using magic, i think it's already used in a lot of stories that things like this are like common knowledge and need no further explanation.
while it's true that holding the wedding ceremony in a church with an audience might be counterproductive to what they originally wanted to achieve, there would more problem later if the wedding were held completely private. They couldn't just suddenly announce he has a wife, the rumours would even be harsher. Some audience, more importantly his people, are still needed as witnesses. And i see that they didn't invite a lot of people.
And from what i see, Ivona no longer cares about her father and she wouldn't care people knowing her bad relationship with him because in the end she's the innocent one.
if later people know her father abused her mother and neglected her, the blame would all be on him anyway. She has nothing to lose, especially since escaping from her father, Tristan, and Gaspal is the most important thing to her right now. Being with Duke is better, even
if somehow people side with her father and rumours are against her, in the end they are all just rumours.
In the first place her proposal was so he could shut people off from bugging him about the whole 'imperfect because unmarried' thingy and so she could do her Kiridan fruit research and get her family and Gaspal off her back.