I as a native german would say in this case it's equal. You can use both.

Unserer is dative - as in "Wir spielen mit unserer Katze" but here there's no dative. Hence unser for singular neuter nouns.

In this case there is no difference. But "Zimmer" is only used when you mentioned the rooms in a flat or building. "Raum" you can also use for this but it can also be used as translation for "area", "sector" and "distance". For example "the distance between us" (der Raum/die Distanz zwischen uns), " at the policitcal sector of Hungary"(im politischen Raum von Ungarn), "in the area of London you find a lot of footbal fans"(im Raum London findet man viele Fußballfans).

After verbs like "to be", it is not accusative but nominative. It is an attribute, not an object. It is not something on what the subject do something, but it is equal to the subject. Here "Das" means, equals "unser Raum". And the form in nominative masculine is "unser".