I don't know if anyone has brought up this possibility yet, but it seems more than likely to me:
All the guesswork about Cartman's father is a waste of bandwidth. The question will never be resolved. They will just leave it hanging, and go on with a completely new episode.
Given the past 13 shows, I find it impossible to believe that Parker, Stone, et alia would devote a whole second episode to the resolution of the paternity question. Not enough action, too "character"-driven, too soap-opera-like, not enough potential for exploring new avenues of shoxploitation humor. And the "revelation episode" is scheduled for April Fool's Day, no? What better way to jokily screw over your audience's expectations than by not resolving a much-publicized cliffhanger?
I could be wrong, of course, but I'd like to go on the record now with that as my guess.
Warmly,
Adam