Was Paul Bonacci the first...

Only God implies first.

the first to come forward about split personalities through ritual abuse

Effect coming forward within cause implies the split of per (by) son (sound) al (all)...it's taking on personas like names; ranks; status; order; caste etc. which represents the abuse of ones rite of passage.

One first has to fill ones mind with personas to be able to switch between them...further abuse from outside tempts one to hide inside ones mind by switching personas.