Yobi can become human by stealing a human soul. She does her best to insist she doesn't she isn't interested because humans are dumb and silly when others bring up the possibility, even though it's obvious she enjoys being apart of the human world.
It is unknown what would have happened to the Aliens that served Yobi as a family now that she had died.
The Fox Hunter and Mr. Shadow are the two main antagonists of the film, with both having different motives: The Fox Hunter wants to end Yobi's species since they're evil, while Mr. Shadow wants to use Yobi to find a Heart for him to use.
The runaway alien would not fit in so well in normal society.
It's not possible to take a soul from the lake without replacing it with a different soul.