>>/10705/
> calling them out only to retreat back into distracted book mode when the tree had befallen her house felt a bit like her holding the idiot ball. 
The irony of that moment to me is I love Twilight doing stuff like this in the early seasons, but it laid the groundwork for neurotic moments like that to define more and more of her character as the show went on. At this stage she's perfectly intelligent and capable, it's just that her obsession with books sometimes becomes her sort of nervous retreat. Which is how real obsessions work - it's a common joke to call her autism horse but having autism myself, special interests are kinda like that, they're just part of your life most of the time but in panic situations you can find yourself just shutting down and obsessing over it to calm yourself down. But Twilight being a fully capable person who just happens to have obsessive and anxious tendencies is tricky to balance from a writing perspective, and I almost feel that with any long-running show (see: jerkass Homer) nuances like this will inevitably get flattened. Mind you, my memory of Day Zero is that even in this early part of the show, they take it too far sometimes.
 >>/10706/
> The mass of those clearly is not the same as the tree that was. I'd like to say that a lot of that mass was used to fuel the spell itself but Twilight conjuring a door just two Episodes ago, if even Rarity's own previous use of this ability this episode probably dispute this. 
I feel like we've seen Twilight just "poof" things out of existence, though I can't recall any specific examples. But in my mind at least it's maybe that it's what you can best visualise doing to the tree - Twilight obviously is a master of magic so can just imagine it disappearing, but I could easily see pretty much all other unicorns having difficulty with that, and it makes sense for the easiest thing for Rarity to visualise being a transformation into orderly shrubbery. Whether it's a matter of power as to how much matter you can displace, I don't know. What would the relation be between intelligence and raw power anyway? So far we've seen a pretty much exact correlation, but to me I'd imagine that intelligence determines how effectively you can use your powers rather than their strength.