To your point about Klaus coming back for a human Katherine's doppelganger blood: Would it have worked, though? She had the cure in her system, which canonically will turn any vampire human if they fed on her, so would that somehow work against the turning process, like it was tainted?
It would have worked yes because you need the WHOLE cure to reverse the immortality/Vampirism thats why Silas had to drain her completely for it to work. Thats why Elena couldnt just take a sippy sip from Katherine with Damon and turn human to frolic in hell with the horrible relationship that is D.lena, and why Stefan also couldnt just sippy sip off Katherine to become human.
You need considerably less doppelganger blood to create a hybrid.
He would have had to do it SLOWER bc he couldnt just heal her with his blood when he drained her pretty hard for blood bags- but he also would have just taken her and wouldnt have needed a stock pile.
If all you needed was a sip of the cure then a lot of problems would have been easily solved but you need the WHOLE cure to reverse the immortality.
But it still would have been incredibly dangerous bc she also got clotting problems as a human, she bleeds a lot longer than she should (as mentioned canonically). Though I would be curious to see how his original hybrid blood would have reacted since his is not normal vampire blood theres every chance it didnt get rejected the way that regular vampire blood did. He very well could have been the exception. He still never would have turned her again bc she was more useful a human than a vampire. Or at least I ASSUME thats more up to @klaeus to confirm or deny- but from the Katherine side of the fence I doubt he would have ever turned her again unless it was mega important that he do.
