Does "layout" include filtering out some (kinds of) properties?

In general the article placeholder will likely not display all the data Wikidata has about an entity. Deciding which should be at the discretion of the local community if they so chose. There will be a sensible default for the small communities we are primarily targeting with this and who might not have the manpower to come up with all the Lua modules and templates themselves.

Unrolling hidden portions? That involves regrouping and resorting of entries.

Note that the interpretation of "editing" changes depending on the mental model conveyed by the page itself. It now lives somehow between the item page at Wikidata and the article page at Wikipedia. Because it is on Wikipedia (ie the client) it should convey a model that supports "this is an aspect of the the article", not "this is the Wikidata item page at Wikipedia". I think the later one will be very confusing, the whole concept with Wikidata already creates a lot of confusion. ;)

I don't see how this was resolved. The explanation would be suitable for the to-be-created https://www.mediawiki.org/wiki/Help:Extension:ArticlePlaceholder