So I solved my problem by scrapping the original code. Based on Alex's comments I don't know if it was ever going to be workable for any situation that had more rows in a dataprovider than there were onscreen rows in the datagrid.
It is definitely a problem with mindset though as the solution that works, and works well, is utterly alien to my old way of coding. That said, would it kill adobe to add methods to style a row in a datagrid?
