I rewatched LBT4 earlier today, so I feel a bit more confident about consolidating my thoughts for Ali in this thread.

1) Personally, Ali—at least in
Journey Through the Mists—and her initial apprehensive closed-mindedness was a foil to what Littlefoot and the Gang of Five stands for. Her unwillingness to play with non-longnecks and unintentionally causing a divide between Littlefoot and his other friends (especially Cera) harkens back to Littlefoot's mother saying that longnecks never play with any other species all the back in the first film, and the entire premise of Cera's jealousy over Littlefoot's new friend in fact repeated in a lesser way for Mo in
Journey to Big Water (another film w/ Journey in the title, ha).
I will say that I did not remember her being so spunky and headstrong towards Littlefoot, though. She bites down on Littlefoot's tail and impishly hides in her first appearance. She comes off as cheeky, firm, and rambunctious to Littlefoot (I can see why people like this pairing from her moments), though meek to everyone else as she cowers beneath Littlefoot in stark contrast to her opening scene. Her opening up to the Gang is one of the highlights of the film as a whole, especially as it's kinda a non-issue by the TV series (a separate can of worms). She would definitely have benefited from a reappearance in he longneck related films, say X for instance.
2) This is a joke, right?
Journey Through the Mists, of course, unless you mean the Ali from the start and end of the film. I'll say, a long-term narrative of her turning from distrustful Ali to open-minded Ali, instead of the Archie cave sequence forcing her to trust the rest of the gang in such a contrived way within like maybe 10 mins of film time, might make for a compelling arc, ala a TV series arc.
But overall, that's my thoughts on Ali. Definitely a hard character to nail. As I said, her character seems all over the place because it slowly changed throughout LBT4 and then shifts completely again in
The Brave Longneck Scheme, and this is exacerbated by fanfics (especially romantic ones) usually selling her spunky side short, but she definitely is a great if underutilized character. I can see why so many wanted her to join the Gang, that's for sure!