Addressing the way they used the elephants, it really wouldn't have made sense for them to speak given that they're portrayed as being almost the deities of the jungle. Having them talk would've removed some of the 'otherworldliness' that the movie associated with them, and made the scene where Mowgli befriends them and the scene where they put out the fire by rerouting the river less impactful.
As far as Kaa goes, I would happily have taken more of her, but it's not a detriment to the film that she doesn't have more screentime, because what we got of her was perfect, and it actually made more sense, contextually, for a friendly character to rescue Mowgli from her than for Shere Kahn to do the same, at least in terms of the way in which they restructured the story.