There are people around here that are infinitely more qualified than me to answer these questions, but here are my very humble opinions:
Outliner notes, I think. The plugin would have a whole file just for itself, with no risk of ever getting it ruined by an improper edit on the embedded content.
I know what I would like: infinite nesting, hoisting, compatibility with Obsidian’s wikilinks and block reference.
Probably not. Anyway, I think a plain text outliner would be a minimum viable product.
I’ve seen outliner exports relying on nothing but indentation or very simple syntax, so metadata shouldn’t be a dealbreaker. As for OPML, it is indeed a standard, but a pain for readability.
I don’t think 2 MB is an insane amount of text. But then again, the devs should be able to give you a clearer answer.