what type of cell form endocardial cushions? and what happens when there are defects in the proposed cell?

neural crest cells

there will be atrioventricular septum related issues and also possible facial issues as neural crest cells help to form the cartilaginous precursors for the face so usually heart defects and face defects can be related in this case

the reason atrioventricular defects may occur is due to the fact that neural crest cells make up/produce endocardial cushions and so without these the AVS cannot be formed properly