I mean, aside for her misdirected concerns about Brian's virility, Carol seems to be the sanest person in the strip and certainly the sanest person Elly has ever met. Sadly, this might present something of a hindrance to their friendship. Carol isn't some clingy, man-hungry, idiot hypocrite willing to throw her children under the bus who boasts about how loving a mother and self-sufficient a woman she is or some pontificating, preaching dough-head with her head in the sand about how bad off she is so that's something that Elly can't like about her. After all, Elly is looking for someone to both be threatened by and feel superior to when she wants a gal-pal; Carol is just too good a person to fit Elly's weird needs.
Similarly, Keith might indeed have gotten John interested in model railroads but he isn't an obsessive loon who uses the hobby to hide from his wife and children. The reason that we don't see him in the later years is that he's got better things to do with his time than hang around some dozy prat ranting about ungrateful children and being tricked by George Stibbs. My guess is that as I write this, the two of them wonder what the Hell John and Elly think they're trying to prove marrying their children off to a pair of yes-men or babbling about how their children owe them a living.