Could you help me understand why so many people think Drogo was Dany’s Nissa Nissa? She smothers him with a pillow in Dany IX, and in Dany X there is not a single mention of signs of “life” in his body – she’s preparing his truly-dead corpse. When she’s talking with MMD about blood magic, Dany says it’s HER life she wants as payment, and she hates MMD. The eggs then hatch as MMD is dying. Drogo’s body was… there. So where’s Nissa Nissa? Could it be that part of the legend is wrong?

I would not draw a direct line between what happens between Dany VIII and Dany X and the Nissa Nissa story, necessarily. GRRM describes it as a miracle, a sui generis event.  

Because there’s a lot going on:

• Dany agrees to a blood magic ritual to heal Drogo, and Mirri Maz Duur tells her that “only a death can pay for life.”
• Dany is taken into the tent where the “shadows whirled,” which Mirri Maz Duur warns her about, saying that “once I begin to sing, no one must enter this tent. My song will wake powers old and dark. The dead will dance here this night. No living man must look on them.“
• Dany’s child is slain by magic. While this could be due to the accidential exposure of Dany to whatever the hell was in that tent, Mirri Maz Duur takes credit and claims that Rhaego’s life was the cost for Drogo’s resurrection: “no, that was a lie you told yourself. You knew the price.” Then again, Mirri Maz Duur has been lying to Dany all along specifically to prevent the prophecy of the Stallion Who Mounts the World from coming to pass and to take revenge against Khal Drogo for enslaving her people.
• Dany conducts a blood magic ritual based on whatever clues she’s gotten from MMD and from some unconscious urging. (Notably, when Dany awakes from her birthing bed, she’s already crawling towards her dragon eggs, with the words of “waking the dragon” ringing in her ears) 
• This ritual should not have worked on its own. As MMD says, although possibly trying to save her own life, “by itself, the blood is nothing. You do not have the words to make a spell, nor the wisdom to find them…loose me from these bonds and I will help you.”
• This ritual definiitely involves the sacrifice of Mirri Maz Duur: ““it is not your screams I want, only your life. I remember what you told me. Only death can pay for life.”
• This ritual also involves the ritual burning of Khal Drogo’s body and the three dragon eggs with him: “She climbed the pyre herself to place the eggs around her sun-and-stars. The black beside his heart, under his arm. The green beside his head, his braid coiled around it. The cream-and-gold down between his legs.” Note that Drogon’s egg came from Drogo’s heart.
• This ritual also definitely involves a great heat as well: “she was the blood of the dragon, and the fire was in her. She had sensed the truth of it long ago…but the brazier and not been hot enough.” Remember, the sorceries of Old Valyia were “which were woven of blood and fire” (perhaps the words of House Targaryen are a secret riddle as to the source of their power?).
• Finally, Dany also walks into the fire, bringing with her the blood of the dragon, the blood of Old Valyria. 

Which elements of the ritual were necessary? Which were not? It’s hard to say when you’re talking about something that’s deliberately non-standard, and when the author is deliberately not systematizing magic in order to keep it magical.

What we can say is that this bears little resemblance to Nissa Nissa. Dany didn’t give a damn about Mirri Maz Duur, Drogo was already dead, whether the eggs were alive is difficult to say since they come from Asshai and that place’s relationship with life and death is borked. But certainly MMD didn’t sacrifice herself voluntarily, and it’s hard to say whether Drogo and the eggs count. 

However…Dany might be considered a willing sacrifice.