Thanks for the questions and for outlining the toolchain constraints. Let me answer them and clarify what the change does.
In particular, is this changing the default HTML that's immediately available at the root of a DocC archive, or is this changing HTML that's at the /documentation subdirectory within a DocC archive?
The proposal replaces the current “not found” experience at the archive root (/) with a curated hero and top-level cards. It does not change the content at /documentation; that page continues to render exactly as it does today.
The renderer reuses the existing index data that DocC already emits. It simply filters the navigator tree to its root nodes and renders those as cards alongside the hero. No new DocC output files or alternate routes are added.
If a DocC archive has multiple modules, how it that presented?
Each top-level module becomes its own card, so a multi-module archive shows one card per module at the root. Tutorial bundles or other top-level navigator entries are treated the same way.
Would this help that situation and provide links to the /documentation page with its panels?
It would surface links into each module’s /documentation/... page; it does not change the /documentation page itself.