As for plot wholes, I think SeanzOz was pretty close, if you don't know want I am talking about search "foolhardy Skyrim part 9". Now even if the bow was not going to really blot out the sun completely, the plan never would have worked. Vampires overrun the whole of the continent during a cold war, with fire breathing lizards in the skies, give me a break. I might not like the Thalmor but they are more than capable of moping the floor with Harkon's lot.
Harkon just wanted to block the sun so that vampires could go about as they pleased. Whether or not he wanted to take over the world (which, if he were sane, he could do by enthralling a few choice figures), his plan wouldn't have been good for mortals.
Then kill her or leave her description with every inkeeper and guard in Skyrim or do anything except help her. What would the vampire hunters do about the ancient and difficult to find Daedra blessed vampires? The dawnguard as it stands at that point isn't much smaller than the dawnguard strike force that sacks castle Volkihar, in a stand up fight they could take Serana. If she's disinclined to talk take those torture implements for a spin, maybe that's not your dragon borns style but it's something mine would do and it's apparently something Isran was ready to do to.
Appearance doesn't mean all that much in a world of Illusion magic, and Serana doesn't need inns or civilisation; she just needs to get back to Castle Volkihar. If she does so, then the Dawnguard knows nothing about her, her plans or Castle Volkihar. The Dawnguard at that point is Isran, Durak, Celann, Agmaer and you: assuming you're unwilling to murder Serana out of sheer force of racism (and if you're willing, refer to my "psychopathic murder of Delphine" argument) then that's three soldiers and a farmer vs. a Vampire Lord who's just trying to leave. One bat trasformation could get her out.
As for torture, if the Dawnguard and Dragonborn were willing (which I'm not sure I buy), and if Serana were incapable of escaping, then Serana could always just lie or claim ignorance. Torture is not actually all that useful for information extraction, especially in open-ended questions (it's more useful for obtaining confessions), so there's nothing to stop Serana from saying that she has no idea what Harkon might be planning, especially considering that she's been underground since before the Alessian Empire. If she needs time to make up a story, that looks exactly the same as if she just took time to break.