>Gesture and setting are an important part of dialogue, but the tone I'd set for the story was fairly stripped back up until this point, and so I tried to continue that by stripping my dialogue of all but the most important physical cues.
If it were a short conversation, it wouldn't be much of an issue, but it does add a healthy touch of realism. When you're talking to a friend, do you notice him gesturing with his hands? Do you notice what face he makes in response to what you said? You want the reader's experience to mirror what real life would be like as much as possible. And just like a stock portfolio, diversification is key, in this case, diversity in the means used to convey information. We call this "talking heads," and certainly some readers are more sensitive to it than others. It wasn't horrible, or I would have mentioned it earlier, but it could use a bit more.
>And I have to ask, is it necessary?
Maybe? All I can say is that it bothered me. You made a point of having Apple Bloom there, and then I kept wondering when it was going to matter that she was. As much as AJ tries to take on the burden of the situation because of how it's affecting the rest of the family, it seems like she'd notice how Apple Bloom was reacting to the argument with Granny Smith, even if Apple Bloom keeps quiet.
>I figure she takes it off to sleep, just as Applejack, later, takes off her hair ties.
Could be. Though in "Somepony to Watch over Me," Sweetie Belle wears the bow as a disguise, and AJ wasn't surprised by it. She has it on during their sleepovers, too, in "One Bad Seed" and "The Stare Master."
>Could you explain the difference, please, or at least help me understand it better?
Well, it goes to the differences in narrative voices. An omniscient narrator has to present thoughts as quoted (or italicize