Playing dawnguard last night and couldnt find answer to these questions

If skyrim scholars interviewed gelebor would he have been able to provide more details on dwemer culture etc? Is he immortal? What about knowledge of other creatures such as giants etc?

Dwemer vampires/ghosts? Any records of these? Could they still exist even though dwemer are gone?

Dwemer ghosts existed in Morrowind, but like Gelebor, we only spoke to one and we weren't given the option to ask him that much about his culture. Just wait until TESVI:

Steve: Greetings, %PCRace! I am Steve, last of that otherwise extinct race native to this province. Fortunately, I fluent in your language. I'm sure you have many questions and I would like my account to preserved by your civilisation.

%PCName following the only dialogue option available: I could care less. Just tell me how to get the MacGuffin.

Or we might get a book about Merethic Skyrim written by a someone who did what you said.

Still, it's curious that Morrowind's ruins are haunted if Skyrim's aren't.

Morrowind had Dwemer ghosts but they were hostile as for vampires I think vampires didn't come around until after the dwemer but I'm not sure. I imagine the betrayed attacking the Vale was more recent then happening in the first era, I don't think Gelebor is as old as some people make him out to be.