Huh, it seems the poll options are alphabetized... and here I thought I accidentally showed favouritism to my choice by subconciously making it first.
Personally I dislike the changes. While I'm all for Glasya's increased importance, it really should have been better explained. Asmodeus does not do anything lightly, especially trusting his daughter. Also, such a plane shattering event as the death of an archfiend and a complete redecoration of a planar layer really deserves its own adventure. All it got was a few short paragraphs that vaguely hinted at a coup. Such mysterious nothingness puts the writers of Lost to shame. Also, none of the Archfiends have such total control over the layers over which they rule that they could deform the entire infinity thereof at a whim. Even deities have finite realms, so Glasya shouldn't have been able to abolish the mountainous nature of her new home with a grin and a wink. Overall, while I'm not against the idea of a change in power, it should have been well described, with loads of potential PC involvement, and followed at least some of the rules unless given a reason not to.
Personally, I don't mind the reshaping of Malbolge. Before, it seemed to be a layer that was poorly thought out, as if the original designer was running out of ideas. After all, the idea of an infinitely sloped plane is just next door in Gehenna.
In addition, I never liked the Hag Countess as a power player in the hells. She always did seem like a "placeholder" - again, it seemed the creators had run out of ideas to make her unique.
As far as who is responsible, I figure that Asmodeus could have orchestrated the whole thing himself, or used some sort of power pact. He could have even duped a group of adventurers to bring him the components he needed. Perhaps Cegilune helped (nice idea).
There is already evidence that Asmodeus can force a devil to change its truename. Asmodeus could have simply changed the Hag Countess' true name to "Malbolge". Heh...