There was a single book in the game world, hidden away in a vampire's dungeon. It was rare to begin with. You think the Order is going to waste time sitting down with new members to tell them how everything goes? They have an established system, and the book was written to further educate possible new vampires.
When you have a society of great secrecy yes you sit them down and tell them. The order is supposed to blend into human society not live in holes like animals.
Or maybe the guy who wrote the Vile Lair didn't know anything about daggerfall and simply made a mistake in the writing.
The Cyrodiilic vampires could be multiple blood lines. This much I agree with you. They could of been the Daggerfall vampires built into one single society. And the book itself implies their creation predates their arrival to Cyrodiil. So there is a chance you are correct.
Its the only way to reconcile the fact that in a previous game we had nine seperate clans that could blend into human society with the claim the ability is unique to Cyrodillic. But those nine clans used a different mechanic you could always maintain your human guise feeding everyday was a requirement if you wanted to rest or even use the wait function.
The manifesto is trustworthy. Gameplay in Oblivion is what Immortal Blood says it is. You feed to appear human-like. Also, the vampire progenetor was also written down in the Manifesto as well, Lamae Beolfag. But I suppose that might be another far-fetched and false tale too, right?
Immortal Blood could have been written by a Volkihar vampire who didn't want anyone to know his kind could blend into human society the way Cyrodilic did. Gameplay in daggerfall conflicts with information written about them in the Vile Lair. Lastly its much easier to hide a lie within the truth then to sell complete BS.
The last possible explantion is the books are simply out of date, the manifesto assuming its author was Greywyn would have been written around 3E 321 when he build the Lair you find it in.(though it could be far older). If the Manifesto Cyrodiil Vampyrum and Immortal Blood were penned two centuries before Oblivion takes place it make the information rather dated. But as it stands the information was inaccurate from the first day we read it.