I think the issue really is that Katie wasn't ever legally 1st Prince(ss).
She accepted Yvonne's invitation to take over as Regent.
The real issue, as I see it, is that w/ all the people around her, no one could talk Yvonne out of stepping down.
One would think that Tancred, Ardan, Jackson, someone, anyone, would have tried to do so.
That said, as we saw, Katie had fans even in the Davion side of the FC & yeah, Possession IS 9/10.
Especially if/when it is only legally as Regent & then the real prince decides to become a gardener when he comes back & doesn't bother with going home & putting things in order.
Legally, per the laws, she was still the Regent regardless of what she claimed & was put in that position by Yvonne who was stressing out as a 20 year old who was way beyond her depth in the role.
The bigger issue to me is that Victor didn't fix that situation the second he got back.
Given how popular he was at the time he could have retaken the throne on New Avalon w/o much of a fight.
Oh Katie would have fought, but the troops at that point? That I'm not so sure about.
It's one thing for the 3rd RR to fight in 3065-66, but in 3060 if Victor, Yvonne, Tancred, & several RCTs showed up.
I'm doubting that C* gets involved & the 1st DG was Loyal.
Point is, letting that situation fester for 2 years made the FCCW possible.