Sorry for my layman explanation, but there's no dative substantive in those examples you give.
Just the order of substantives appearing in a german sentence, doesn't automatically identify these as accusative or dative substantives.
To cite wikipedia (probably not the best resource here):
Als Test zum Nachweis eines Akkusativ-Objekts dient im Deutschen die Frage Wen oder was?
That would match the terms
"Zimmer" in your examples.
But I can't spot a dative case, which would be defined as (wikipedia again)
Für den Dativ wird im Deutschen die Frage Wem? benutzt.
The substantives appearing in those parts
"in einer WG" aren't dative subjectives, but merely additional attributions to the accusative substantives, and thus naturally are placed after the related accusative substantives.