I think the logic of not allowing subpages is so Talk:Earth/Proposed_draft can stay a sandbox.
That means:
* For occupy namespaces, it is not enabled by default, so you see a blank regular page, rather than a blank Flow board.
* For batch wikitext->Flow conversions, it shouldn't get 'converted' (moving it to an archive page and/or putting useless content in blank Flow board's header)
However, the below should be fine:
* Special:EnableFlow, since it's explicit
* LQT->Flow, since we know it's a talk page
I think a good way to do this is to move it to the ConversionStrategy (e.g. shouldConvert( Title $title ). Special:EnableFlow can then use a subclass of Flow\Import\Wikitext\ConversionStrategy that waives this requirement.
I think the "Skipping LQT enabled page" in convertNamespaceFromWikitext.php should also be moved to there, since it's the same concept.