lol, i think your overestimating her power up, all she got from shadow is simply a mana increase, all other stats such as sword technique and her fighting experience remains the same,sure it would make her stronger,but at the end of the day, even someone with little mana could defeat a spellsword overflowing with mana with just technique alone. Refer to chapter 2.
So yeah, You're expecting too much for a chased princess to kidnap his father in front of the enemy's main camp. Maybe this is a minor spoiler but yeah, she never had the intention to escape at this point, it's not like she can in the first place. So if its pretty much impossible to escape that place alone, its even more harder to escape with someone in tow...