Ever think that maybe "Lord of Lies", "Lord of Destruction" and so on might just be largely meaningless titles? I mean, Diablo is the Lord of Terror and he's just a big red dinosaur... not that terrifying really
Because technically we (humans) cannot reproduce how terrific he really is by any mean. Same thing for the.. "perfection" of the angels/heaven.
When you read the Sin War trilogy, you have a lot of description of some art made by human that would represent heavens. However for Inarius thus are pal copy that do not represent the real might of this place, even if they're exactly how he had described the place to the artist. Basically, a picture could never represent the ... "might" of the heaven, even if you'd take a camera.
Same goes for the lord of hell. In the same trilogy the main nephalem at some point creates a wall of water that acts as a mirror to actually scare Diablo (it's Diablo if I'm right, but it's kinda far in my memories... anyway if I'm wrong someone will correct me.)
Yes, Uldyssian uses his ill described nephlim powers to mirror Diablo's powers (because at the moment he was trying to appear as what would scare his target the most) and something (we're not told what) actually scares him.
Anyway, on topic, I think it's far more likely that if someone is behind Adria (other than Diablo), it'd be Lillith. For one, she was the first one who wanted to use the Nephilim as an army (against heaven and hell). For two, she's also been know to use females as fronts, and for three, she's one of the few demons/angels to be shown to constantly work against EVERYONE else. The evils tend to work together (except for the civil war, but even then they were in teams). They also tend to be focused on defeating the other side, and don't really care about Sanctuary, i believe Lillith wanted to rule Sanctuary.
As for the Belial, I think he really was defeated because 1) he's over confident, like just about every other Evil (Lillith is less so, because she knows she's not the biggest fish in the pond), and 2) The evils (at least in D3, cause they didn't have as much personality in D2) tend to fall prey to their own "trait." Baal basically just destroyed everything, Diablo focused on Terror, and both were weaker for it. This shows up the most with Belial and Azmodan. Azmodan is supposed to be a great battlefield commander. His assault on Bastion's Keep WAS set up well, he just failed to modify his plan when the heroes show up. Why? Cause he's overwhelmed by pride, and is overconfident. His minions act similiarly, like how Ghom was in the keep for several days, but just tooled around in the lower levels eating people, Cydaea tries to toy with you, instead of just up and finishing you off, as one would think she would trying to protect the FIRST heart, instead of just bailing. Anyway, everytime you take out one of Azmodans plans, all he does is say "HA! I didn't NEED that plan, I have THIS plan! Belial isn't overcome with pride, but he's lied to himself. He's convinced that he's fulled you, when he clearly hasn't (look, if you're trying to be human, don't be a suspicously competent, and creepy, human child. Didn't Belial ever watch MiB?). Of course, I could just be reading too much into the fact that Blizzard didn't write the story that well, and oversold the villians and didn't give them a reasonable pay off.
Also, Lillith being the "big bag" fits her modus operandi - manipulating someone else to do her dirty work. All the other major Demons did their own dirty work (as far as we know). Finally, the journal where Maghda talks about the "other" witch seems to me to be implying the split was not...amicable.
Edit: I forgot to mention, another thing against the Belial theory is that Azmodan claims he can here his brothers "from down here" (aka Hell). I doubt Belial would actually be able to full him (as well as Diablo later on). Someone along the line would have noticed that Belial was missing.