Two things: First, the Collectors did have a voice, but it was cringe-inducing and meme worthy rather than scary or menacing. Second, we already had a mysterious, enigmatic villain: The Reapers. The Collectors were always going to come up short in that comparison.
Who said anything about ME3 being about nothing but building alliances? I simply said it was always going to be one of, if not the essential parts of the story (which is exactly what happened). Thematically, it's much more important than the Crucible: The alliance building missions in Rannoch and Tuchanka resolve major conflicts which embody some of the key ideas of the series (synthetic/organic relations, freedom vs. security, etc.). The Crucible is a plot device, and little more. Further, it's unlikely that Cerberus is the only organization in the whole galaxy who would be trying to do something about the Reaper invasion when it's in full swing, and given the reputation of that organization, most denizens of the galaxy would probably pursue any alternative they could find.
ME isn't a character driven story? What is it driven by, then? What is the point of the recruitment and loyalty missions of ME2? Bioware's calling card has always been characters rather than plot, and ME really isn't an exception. There's the old Faulkner saying I like to quote a lot, which is something to the effect of, "The only thing worth writing about is the human heart in conflict with itself." I think there's some truth to that: The only stories I can think of off the top of my head where the conflict is 100% external come from Star Trek: The Next Generation, seasons 1 & 2 (Roddenberry had all of these silly rules about how the characters couldn't come into conflict with each other, since in the future, we'd gotten past such petty conflicts or some such nonsense). Honestly, it was dreadful.
1) The first point is a matter of how you choose to perceive the Collectors. Honestly, I'd say that their idea of a mutated lifeform under corruption from Reapers makes them qualify as possible distorted view of ourselves in the future if we would fail against the Reapers. And as I said about them being an avatar of the Reapers, on their own, they're still fairly inscrutable and voiceless, especially when they aren't echoing their master's voice. Think about them in MP. There's no voice, just screeching, indiscernible shrieks and a constant, buzzing noise of what sounds like thousands of hornets flying in all directions at once. And think of what they do when they subjugate a person, how that person might feel, and what they believe or know might happen. The Collectors are a compelling villain if you so choose to view them as such.
2) This again may come to how we decide to view this; I focus on the more technical aspect of the universe. The alliance building is a means to accomplish my real goal. I think I've told you that before, I'm more of a materialist who focuses on the external reality instead of the internal perception. I think we're at a categorical divide here on how we view this, and one reason I'd want to take Cerberus as an ally (and possibly forcibly subjugate others to rally behind them) is because of how they achieve their results. They aren't afraid to use Reaper technology to achieve their results, and they aren't afraid to break some rules to get results. They have more experience and knowledge with the Reapers than anybody else. To me, I don't see why anyone wouldn't want to join their cause. And if I were in Shepard's position, I wouldn't care. I wouldn't give them a choice.
3) It's driven by the plot and external events. Character interactions are an aspect of the games, but not necessarily integral ones. Everything is externally driven. You have the Reapers and their prospective return. Then you have the Collectors and their attacking of human colonies. Then you have the Reapers returning and building alliances. The purpose of those missions is to build a team to take on the Collectors and stop their attacks on the galaxy, and deny the Reapers a tool to be used as well as the creation of a new Reaper. That's simple enough, isn't it? That was the point. It's all external. Honestly, I'd disagree with the assertion that ME is any different. I believe ME was an experiment to try a separate approach. Are the character interactions still integral? To the experience, possibly, but not to the plot itself. You can't really shape the plot based off of character interactions except for perhaps the Suicide Mission and the loyalty checks that it relies on. Everything else has advances in the plot taking place over time and completion of external objectives. Shoot, that's not even really new. Every BioWare game I've ever played advanced the plot by focusing on external actions taken by the player or another party. What I'm saying is that while talking to other characters (specifically squadmates) is a big thing for games to a lot of people, it doesn't advance the plot or make the story run faster. And, this is my view, the point of a story is to finish.
I disagree with Mr. Faulkner. I don't think he's got the answer quite right. I'm not going to say that I do either, but then again, answers are endless in quantity. I'm a guy who prefers external conflict in my plots. And I got a feeling that this is going to be an apples and oranges debate as well. I can think of several more, as well as the vast majority of real conflict in real life. And having a significant experience with that has caused it to shape me to be much more receptive to external conflict in stories. For example, Halo is a story that relies, for the first trilogy, almost exclusively on external conflict.
And as a Star Trek fan myself, I can tell you that there was a lot more problems with the first few years on TNG than just the seeming lack of interpersonal problems. But that's something for another discussion.