Well, I agree that living with your partner for a long period of time won't automatically guarantee a strong marriage just because all people and their life situations are different. Here's Shoe's case: she still lives with parents(June's 26yo now) and she is probably not adapted to living without them. She even confessed that she never tried to do taxes by herself. June can try to be big gurl and at least clean her room and cook meals for herself, but her parent still will ununintentionally do those things for her sometimes, since their daughter lived with them so long. And Shoe can't even clean kitchen after cooking with her untreated ADHD, so who needs her help anyway?
If Greg can manage to do all work by himself for some period of time and help Shoe adapt to a new living - they will probably have a fairly normal relationship. If he's as emotionally immature as June and way too hyped about their future life, he'll probably have a mental breakdown. Even if it's possible for him to handle June's annoyingness for two weeks, there is a chance that dealing with her 24/7 will be too much.