Couldn't the antichrist be a collection (so to speak) of people...the conciliarists, or modernists that turned the church upside down?
According to Scripture and Prophecies, no. The antichrist will be a Jew born in the middle-east. He will be possessed by satan and will claim to be either a Prophet or Christ Himself. Now, there will be many people before he comes that will simply pave the way for him (Obama is a prime example), but the antichrist himself will be a single man.
And does it mean the gates of Hell have prevailed? I don't think so, but that is the favored and popular argument that is used to provide an explanation, and a reason why one should not to fall into sedevacantism....
No, the gates of hell have not prevailed. They have certainly tried to, but they haven't. If they had, the Traditional Latin Mass would have been completely destroyed, along with Tradition itself.
Sedevacantism would not mean the gates of hell have prevailed, either. The Church can still exist without a Pontiff. Whether or not one believes Benedict is a true Pope is up to them as an indivisual and is a controversial topic.