I think that although it would’ve been better if she had been able to return to the past before she neglected her son would’ve been great but I think it’s good too that she didn’t. In the sense that this way she can make it up to her son instead of having had a timeline where she neglected her...
I think the prince stabbed her because he has been indulging her for quite a while and i. the subjugation he saw how his actions almost cost the lives (not sure if that one guy died) of people like one that was so important to him and in this case, I think he has tried to talk to her properly...
Maybe it's because of her hesitation to say yes immediately the first time around during the wedding? Might've caused a misunderstanding which snowballed