It is one very tough journey. Keep in mind the MIBG therapy you got is not official protocol yet, afaik it is a new trial arm on COG, as with most NB treatments we are all participating in one sort of trial or another.
My guess is that they will recommend continuing with the COG protocol so an operation would be next followed by a tandem transplant, radiation and immunotherapy.
I would recommend you try contacting St. Jude's and Memorial Sloan Kettering, both hospitals have access to stronger immunotherapy drugs and may have some advice. Additionally, I would recommend you contact CHOP for another opinion on genetic based treatments.
Its a long journey, wishing you lots of good luck, stay strong.
How old is your son/daughter?