Since I have seen so many comments about socks and shoes for Abbie, I would like to share my opinion. Even "normal" kids, many of them have issues with socks. It is a common problem, they don't like them, they feel weird, they throw fits, they tale them off...so you pick your battles.
I think it has been mentioned about Abbie needing to feel the ground...to feel grounded. Like she is unsteady on piers and elevators etc. I think the taking off shoes grounds her and honestly with everything they deal with in her behavior, that would probably not be worth the battle.
I bet her having the slip on shoes makes it easier on the entire family and Abbie as well. Much worse going on there than Abbie's barefootedness.