It is definitely not Blair, who refused to re-do any of the demon makeup from the original film.
Nor do I believe it is Eileen Dietz, because by then she was “big” enough to be credited, and (matter of taste) her facial structure is very different from the mystery actress who plays the young Regan in Heretic. The only accrediting for the young Regan in Heretic is one “Karen Knapp” – who, however, only played the voice of Pazuzu after a previous male actor's voice was deemed inappropriate. Sorry I couldn't be of help, maybe some Heretic scholar could fill you in.