Well, in all honesty, I have never been carded when using the Disney resort transportation system. I think you would be fine there. Showing ID before being allowed on the Disney buses is a rare occurance.
While I understand your dilemma and decision to omit a family member from the reservation to save money, by doing that you are violating fire codes and could get in trouble if you were to get caught. That being said, I don’t think your chances of getting caught with an extra person in the room are very high - and if they were to catch you, you could probably write it off as if you were just having a visitor over.
Like you’ve said, you will not be able to do EMH without someone sitting out because they card you at every attraction. Magical Express would also not work in this scenerio. The only other thing that might go wrong is if a park reaches stage one capacity - normally resort guests are still allowed in with their KTTW card. However, if the park is that crowded, I doubt you’d want to go in, anyway.
Now, if you want to book an inexpensive room for your family of 5 without lying about who is sleeping in the room, Port Orleans Resort Riverside (Alligator Bayou) can sleep five people with a trundle bed and isn’t much more expensive than the value resorts. This would give you a bed for everyone and some extra floor space. Other options are the cabins at Fort Wilderness and the family suites at All Star Music and Art of Animation resorts, which would give you two bathrooms.