So Valerica stashes away Serana and then goes into hiding herself so the Harkon can't get the blood of a daughter of Cold Harbor. Fine as far as it goes. But when exactly is this hiding part supposed to end? And how? What's the plan here? Stay hidden FOREVER?! Why not just commit suicide? I don't get it.

Waiting for the Dovakhin?Dawnguard to come along and trash Harkon for her perhaps? Sometimes survival is enough of a plan?

I can add to the confusion. Why if Serana is locked away with the scroll in that dungeon to protect her from Harkon, the first thing she want's to do is go and put herself in harms way by running home? If things didn't go to plan like she says then a prudent person would assume that Valerica met her end at Harkon's hands already, there would be no reason to suspect Harkon would not be waiting at home.

And to further this, after all that became understood, why not just kill the daughters to end the plans? If you sided with dawnguard and truly wanted to be a vampire hunter it should have been done right then and there and take the scroll away.

And why wouldn't she just wait in soul cairn and trust you to kill harkon? Or better yet, lock the bow in soul cairn lol.

The bow is weaker than other bows so it isn't necessarily useful to use in that final fight. And it's the main thing he needed.

Neither would fully stop Harkon. If Serana and Valerica died, he could just find another Daughter of Coldharbor. And with the scrolls in the possession of the DG, it gives him a target to attack. It may slow him down, but time is on his side.

The bow's pretty damaging with sunhallowed arrows against undead. Explosive extra-damage-to-undead sun damage.

Sunhallowed arrows can break the red bubble-shield that Harkon creates, and he heals himself while inside them.

Hiding the bow in the soul cairn wouldn't stop him?

That's the one thing he needs in the one place he'd never find xD

Either way it's taken him centuries I'd assume, just to have his daughter return to him. He hadn't found another daughter since she had left, so I'd assume it'd be a few centuries before he could find one. That delay would last lifetimes and you'd be in the clear xD

It'd be just like with Alduin. Those people gave a temporary fix sending him to the future. Well, they no longer had to deal with the world ending lol

Valerica may also have been planning to harness powerful undead in the Soul Cairn and lead them back to kill Harkon. Probably not but then its pretty clear she was desperate enough to (even remotely) trust the Ideal Masters who are renowned for betrayal.

You know, I'm glad you brought that up as I've been having a hard time wrapping my head around it.