Decision: Made. Mostly.
This morning Seymour and I toured Big Noggin, the local GATE 3 - 8 grade holding pen.
They have a compacted and enriched curriculum; have active social and emotional safety nets for their students; are very much into teaching critical thinking, organizational, and study skills; and have no problem with Iz's being younger than the standard age for her grade, especially since she's been "tracking" since 1st grade. They even have precedent for kids such as her.
Only negatives are that they can't provide official support for Spanish at her level, and that they do not offer languages beyond Spanish. But I am no longer worried about that. Her teacher says Iz can always go study abroad for a year, and a parent who's been through our situation says that we can just get her a tutor with some other friends once every week or two.
So, if she gets in, I do think that's where she'll go. It seems like The School Built for Iz.