if Katara was given a few more moments to use the Spirit Water on Zuko in CoD, that definitely would not result in a Zutara ending.
Consider the following:
Spirit water doesn’t heal scars. Aang’s scar from Azula’s lightning strike wasn’t healed. The spirit water only revived Aang.
Zuko wouldn’t take this lightly. He’d feel lied to and taunted. Finally, as his life seems to be turning around, he’s given this chance to be freed from his past, but it only serves as a reminder that he’ll never move on, and that it will always be a part of him. I think this would only encourage him to betray Uncle Iroh.
Katara would feel crushing guilt. Not only is the world’s hope, her hope, her best friend and light of her life dead, but she’d feel like it was her fault. She could have saved Aang, but no, in a moment of trust, she wasted the Spirit Water on Zuko, the person who would ultimately aid in Aang’s death.
I don’t know if Zuko would be as compelled to go join the Gaang if Aang died. He only started doubting himself when he found out Aang was still alive, and he convinced himself that it was his destiny to train The Avatar in firebending. Without either occurrences, I don’t think Zuko would be motivated to leave his family and home. However, let’s say he did anyway …
Katara already threatened to kill Zuko, and that was even after the rest of the Gaang accepted him. I’m pretty sure that if Aang actually died, the group would let Zuko join, and I don’t have a doubt in my mind that Katara would unleash an unfathomable wrath against Zuko if she got the chance.