The screenshots I have seen of wyverns in the game do have the same shape of head, but as far as I can tell, they do not have feathers. I may have missed a close-up with more detail...
...Oh, and are we sure that the dragon knights are humans who work for the dragons?.
Yes. They may be more like familiars or worshipers, but they are humans that work for the dragons. From the D2:ED site;
Favoured by Dragons, the Dragon Knights are the Old Race’s human agents who have been blessed with the formidable ability to take the shape of their legendary masters.
In less tumultuous days they ruled a Rivellon in relative peace: an image now as evanescent as the dominion of the Dragon Knights themselves. That they will become fully extinct is not a possibility, but a fact, because sooner or later the Dragon Slayers will find and kill all remaining Betrayers and so avenge the fallen Divine.
I really missed bit of the website, apparently.