Yep that's just her throne room. But having her move over to Beelzebub's is actually a pretty interesting idea, I'll have to give that some consideration!
Not to be a buzzkill to the Beleth fans, but she's likely going to be staying the same size in Impious (though by good buddy H-Clam has requested some Beleth growth stuff over on dA! There may be more non-canon Beleth growth stuff later on). The general idea being that growth/size = power, and Beleth begins the game towering over the imps in every way. But this natural size and power means she has neither the need nor interest in "self-improvement," and I feel it would be more satisfying for the imps to come back and wind up being larger than her in some way (even if it's just having a larger chest or butt or what have you). I'll give it some more thought, though!