What if in the Three Siblings AU, Chara tried to do the plan to break the barrier with Undyne instead of Asriel? How would that go?
Oh boy, that would go pretty different! One of the main points of the (tentative) canon for the AU is that the original plan didn’t go through, and Asriel and Chara were able to grow older and reach the point where they met Undyne. But! I think this possibility would be fun to figure out too! (And by fun I mean sad!!)
If Chara went directly to Undyne about it, she’d probably agree to it like Asriel did. But I think Undyne was more willful than Asriel as a kid, so when Chara started getting sick, there’s a good chance she’d bail and tell Asgore and Toriel. (I’m not sure if that would change Chara’s fate, but I hope so!) If she didn’t bail, the next sort of divergence from the plan would happen in the human village.
Undyne still isn’t super on board with humanity in this AU, but she is a kid, and I don’t think she’d actually want to kill anyone went the time came. But she’d probably be fine with self defense and fleeing. Chara and Undyne would have an argument after they retreated to safety, and ultimately I think Undyne would win it, deciding they shouldn’t have gone through with the plan. Chara would be too worn out to change her mind once she’d decided this.
In the end, they’d head back to the Underground because they wouldn’t know what else to do, and both would survive. There would be a lot of stuff to work through in the aftermath of the plan’s failure, but eventually I think Undyne and Chara would be able to adjust to sharing a body. They don’t return to the surface (at least, not til they’re older), because the responsibility of trying to free an entire kingdom is way too much for two children. So the events of Undertale would probably still come into play, just with a more peaceful Underground.
(Here’s a bonus drawing!)