Uh, disregarding the Herb of Awakening discrepancy, -40% for Hina is +80% for other characters. Might want to redo your tests with that in mind before coming to a conclusion like that...
1)Set up time is not much different, though its likely safer for Hina. You do have a point about the boss surviving the hit though.
2)Byakuren can work with Hina just fine. There's no reason Byakuren shouldn't be immune to debuffs and if you're playing with Hina in the party, you'd have characters that are immune or nigh immune to debuffs already (like Parsee for example). Anyone that's not just needs some debuff resistance and they're good to go.
You did notice I mentioned the huge difference in their respective attack stats as well? I'm well aware of the effect of Curse Reversal, but the point of the quick test was not to set them on an even ground. I just wanted to see for myself if Hina can reach the "Flandre level" of damage with a bit of effort. Since I use Flandre for the end boss gem farm I've gotten her an ATK buff of 80% with herb of awakening, which makes Starbow Break hit for 1,5 million damage.
As for the setup comment, I find Flandre to be faster since you can just copy Byakuren's buffs on her before she even gets a turn, whereas Hina needs to do all the heavy lifting herself to set herself up for the damage. This of course applies only to longer fights you don't plan to end in a couple of hits and where Flandre is waiting safely in the back row for her turn.
I guess the Byakuren comment came out a bit wrong, since the setting I had in mind for the combo was a first turn nuke without dedicated DBF resistances. In a similar setting to Flandre's back row waiting, fully buffed Byakuren with DBF resistance can just shrug off Hina's debuffs and then share her buffs to an ally that hasn't got that high DBF resistance (or got unlucky). Hina can then do her own thing without having to worry about the others and since she's rather bulky after getting some debuffs, she shouldn't be in much danger. Focusing on using a party with high natural/built DBF resistance just for the sake of making Hina non-detrimental to you only applies to main game in my opinion, since for post game bosses those kind of parties aren't likely very effective.
All in all, which character you prefer to use depends not only on the enemy you're fighting but the playstyle you want to have. Both are entirely capable of filling the role of a nuker. Flandre has an edge with bigger numbers and more frequent attacks, Hina has an added benefit of debuffing the enemy in addition to being quite bulky.