That being said, there are other things that she believes to be common knowledge that are not. As by way of example, we are all supposed to despise the fact that Georgia is seven years younger than Phil is. This, I should think, is because Lynn has it in her head that only people who could have attended school at the same time should date because that's what happened in her life. The idea that other people are not her and thus can and should live life the way that is most suited to their needs is as beyond her as the idea that merely because someone is paid attention that's being denied her that that someone is loved more.
Another odd "fact" that just isn't is that Anthony is desirable as a husband because John and Elly know him and Gordon. It doesn't follow logically that his being a known quantity makes him a better husband than someone unfamiliar because they aren't the ones that have to live with him after the ceremony. Where Lynn seems to be deriving that counter-factual belief from is her lack of any real curiosity about the world around her. Since it would never occur to her to look farther than down the street for anything, Liz is supposed to share her mindless lack of interest in the novel and unfamiliar.
Speaking of which, let's examine why any sort of play that requires imagination is bad. Any sort of play that doesn't involve sitting quietly and not making noise not only involves adding to poor Elly's burdens because it makes a mess for her to clean up, she also risks being tormented by questions that she has to answer (and cannot, being an uneducated and uneducatable imbecile) and the revelation that she and John are not owed back the money spent on raising the small ones.
This leads us to the most lunatic non-fact of all: Lynn's insistence on the fallacious notion that John and Elly are decent, respectable people worth knowing. Watching the slithering vermin sob and snicker as they cheat one another almost makes me think that we're actually being asked to revile cautionary examples but, sadly, I know better.