I don’t see a compelling reason for the editing page to behave differently than post or comment.

It's mostly in case of editing entries that are in an obsolete format, or an active format you were using before you realized you preferred a different one -- the old clothes might not fit the new you.

Anyway, I'm leaning toward being more explicit; most appealing thing I've thought of so far is: if you save a post in something that's NOT your default format, you end up on a "success" page that includes a "set this format as my default" button.

Edit: Oh, just realized I didn't answer your first question. 😑 For email and API posts, the user's default format shouldn't apply; we'll use the specified format if there is one, and fall back to a global default otherwise.

Reason being, email and API don't give you any feedback about what your settings are before you hit send. So it'd be like, "hope you remember exactly what you set previously," which, bad! So for automated or semi-automated stuff like that, deterministic consistency is way more important than convenience.