She put the world in danger by taking Varric, the man is supposedly loves, as well as the only person in the world that can seal the rifts with her to lock the door. No bodyguards from the Merchant's Guild? No hired hands? Nope, she decided that the Inquisition could play bodyguard for her. And that's all my character was doing, was bodyguarding this little mary sue brat. She could have locked the door on her own like a big girl with some bodyguards, and then wrote Varric a letter. Just her showing up at Skyhold was dangerous; the Inquisition has enough would-be assassins running around without this spoiled child bringing them around. She only took responsibility for her actions after she had no choice. Even then, she acts like "See, the door's shut, everything is fine now." Even Varric chews her out, stating that she can't just make this problem go away.
So, she needed the Inquisitor for one reason as far as I'm concerned: She wanted to confess her guilt before the Inquisitor found out about it. She lied to Varric about where she was going; she lied to the Inquisitor and again...took months to lock up the door. If she had locked as soon as possible after the Conclave, I wouldn't be angry. As it is, Bianca used the Inquisition to cover for her mistakes, and put all of them in danger of red lyrium madness.
Bianca screwed up, and yeah, lied about it. But she did what she could to make things right.
I mean, Inquisition playing bodyguard? There are War Table missions that are way more menial. And hired bodyguards would just mean more people knowing about this route to Red Lyrium. The Inquisition has far more "need to know" bout this than random dwarf goons. Not to mention random dwarf goons don't know the risks of red lyrium. The Inquisition does, and can take precautions.
Besides which, there are darkspawn, carta, and I believe Red Templars running around there. Might be a little much for Bianca on her own, even with bodyguards.
Showing up in Skyhold is a risk, yeah, but comparatively mild. It's got an army and intelligence network to rival any nation. Skyhold can handle a few assassins. And the information she has, even with details left out, is something that needs to be kept secure. Not put to paper. Too many people know already.
Yeah, Bianca was foolish to reveal the existence of red lyrium. Granted she had no way of knowing the Grey Warden mage she was talking to was Corypheus, but she still should have cleared telling anyone about it with Varric. Dumb move.
Also, she concealed how Corypheus found out about the red lyrium. Dumb move #2. Not to mention unnecessary. save to try and hold onto her pride. But we find out anyway. So much for that idea.