I'm not terribly far in the storyline yet but for the Dawnguard side it seems extra contrived.

One, you meet Serana. You can tell her your been sent to kill her but she tells you she needs to get home, and she has an elder scroll no less. You can't kill her, despite being a vampire hunter and to be there to stop vampires (helping them seems to be the opposite cause).

So you go bring her home, and then go back to Fort Dawnguard where the leader is aghast that you helped a vamprie and helped the bad guys get an Elder Scroll. Yup, I thought that was dumb too. Then he tells you that you and him are the only ones who can be trusted to deal with this.. after you helped the vampires for no good reason other then being asked for help by one.

Next the vampire hunters allow a vampire into Fort Dawnguard because she wants to speak with you. After having already slain other vampires outside of the Fort.

And if her father was so happy to see the Elder Scroll how'd she walk out with it? Maybe that'll be explained later. But the other stuff?

Worst. Vampire. Hunters. Ever.

Like most of Skyrim, give it a chance, it will make sense as you progress through the story

Agreed. That is why I join the Volkihar to be a hero for the vampires, and to fight the evil Dawnguard!

Never will the sheep rise against the lions!

True that.

In desperate times...maybe Isran understood that they needed allies. She did bring her scroll with her so Isran might have seen that and taken as a good will sign. The magic of rp.

You could just go to the crypt firstly. Help Serana. Refuse the gift and then join the Dawnguard - It makes more sense this way. I am not sure if it's possible but I stumbled on some dialogue choices while going through CK and console commands like this one -> Serana: Who sent you here? PC: No one. Who are you?

I always thought of sparing Serana at first to find another Vampire nest. My character probably couldve killed them all single handedly, but it was wiser to prepare later. When Serana came back and explained the situation to me, I knew she could be trusted.

I was really hoping you were going to say something around the lines of "What did you expect from a bunch of Godless, heathen, Protestants?" Entire reason I clicked on this thread.

As to the Dawnguard's methods of escorting their enemies back to their homes when they hold items of near limitless power... maybe they have a strict chivalry policy.

I am not sure. But I think that by just going to Dimhallow firstly, then escorting Serana we would be given the trailer dialogue from Harkon "What is it that you seek...." Anyone tried to escort Serana without any alliagances to the Dawnguard?

Oh, my apologies, here:

Well what did ya expect? Gunmar doesn't believe in Talos, that faithless cur! And the others? They are a kindergarten next to me and my Knights of the Nine. Sure, they existed before us, but we were actually fighting the forces of evil, and believed in the Gods! The only one that has any sense is Florentius Baenius. Now HIS faith is assured. A true believer always talks to his gods. Amen.

nope it doesn't the main plot is full of logic errors like the one explained by the OP.
It ultimately svcks to pretend to be a Vampire Hunter and to actually doe to the fact that the main plot is strict linear help them more than the Dawnguards, thats why you only need to play it through once as a kindred to fully see and experience Dawnguard.