My son was diagnosed with stage 4 high risk neuroblastoma when he was 9 month old and he is now 2 and a half. We finished treatment in February this year and everything is good with that so far. But he also has a developmental delay and isn't able to walk or talk yet. He has other problems like absence epilepsy and a congenital heart defect (VSD) as well (and as a side note: he is very tall for his age), so we induced genetic testing (they did Whole Exome Sequencing) and it turned out, that he has Snyder Robinson Syndrome. He is the first one with this syndrome who suffered from cancer, but there are other genetic disorders which also have an increased risk of developing neuroblastoma, for example Sotos Syndrome (which I would exclude in your case, because overgrowth is one of the symptoms). I just want to point out that maybe genetic testing is an option you haven't considered yet. Good luck for finding some answers!