The volkain are not as mystical as everyone thinks. They maintain this illusion to scare off the Nords who are obviously afraid of anything magic. So they probally don't spend their entire lives living under ice and such and instead of have some way of excessing skyrim from a portal or such under the ice. This could explain why the dragon breaks though the ice in the trailer. As for the bat flying around and transformation into swarms of bats it probally has to do with their current ability to shape shift. These abilities were probally gained from feeding off of the souls in the soul cairn or something like that, thus why they are more powerful than an average vampire. As for only seeming to care about the dawnguard I'm assuming that they actually do care about other factions they would consider enemies but the dawnguard are the only ones who actually know alot about them and possibily posses weapons that can be used against them, thus they the most siginificant threat.
No offence to you.. But i need you to elaborate on this.
-excessing skyrim from a portal or such under the ice- I don't know what you mean. It doesn't sound fantastic. Nor does it explain why a dragon would fly through ice
- Having to do with shapeshifting.. again you will need to elaborate. I am sure that spliitng your mind into a hundred different smaller ones is extremely detrimental and dangerous to learn.
- Soul cairn vampire lord isn't the worst idea you have. It's alright and id let bethesda go on it.
- Dawnguard knowing the most about it? I doubt it. All they have to do is tell the other orders. Furthermore they are currently believed to be a new order and thus would probably be the weakest.