She's not very resourceful or adventurous.

I think the others are more likely to suceed. Even Cindy, just look at her in the third movie! But she would probably be my next choice.

Based on how skittish she acted in the movie... I don't see her really being all that capable....

I feel like she'd be horrible at it because much like Maui, she relies too much on physical violence and brute force to get things done. When she was firing arrows at Mor'du, it's pretty clear that her firing arrows isn't working AT ALL but she continues to do that and run away instead of trying to find another way around it. Moana was chosen because she is physically capable of fighting but was also compassionate and didn't rely on brute force too much. That's why the Ocean never had Maui be the one to return the heart. He would've relied on fighting and couldn't subdue Te Kā with compassion and kindness long enough to restore the heart and return Te Fiti to her true self. The same would happen with Merida. She's all action (Which she's not even very good at) and too impulsive and isn't compassionate, patient, or kind enough. I'd say that only Aurora would've been ARGUABLY a worse choice than Merida.

Cinderella has proven that she is resourceful and physically capable in the third movie. Plus, she has a kind and compassionate heart, as well as A LOT of patience, so Cinderella could actually succeed, though she wouldn't be one of the best choices.

I agree with KataraLover. Merida is too stubborn and has too much of a one-track mind to realize Te Ka is actually Te Fiti. Also thanks to your comment I can totally see Merida and Maui chilling together XD