In the mangadex translation Muzan says that he wont become a demon. I dont know if thats the official translation but i think it was said on purpose so that theory can be ruled out

I definitely think the two main possibilities are

1. What you said about Nezuko burning the flesh and saving Tanjiro, although if that does happen then itll be interesting to see how the battle shifts since if anything it will give Muzan an advantage. All he will have to do is eat Nezuko and then hes basically immortal.
2. I think Tanjiro might get a powerup, but i dont think he will fully recover. I have a feeling that if Nezuko doesnt come and save him hes going to remember the 13th form of the Dance of the fire god through memories/dreams similar to the Rui fight. I dont think he will fully recover if it goes this way though and i do think that he will still be extremely hurt so he wont be able to do much in the fight, or he will last just long enough until sunrise and then die

There is a third possibility, and i wouldnt rule it out. He might just die, i doubt it would happen but if theres any manga/anime i wouldnt rule out the MC dying, its demon slayer