The idea is clever since it provides nice chains of reasoning, but the calculation of digit sums is proving to be very tiresome. I appear to have made a mistake and will need to backtrack but that should not be too hard as I always keep a record.
I have spent a lifetime doing mathematics and I have to admit that I find these numericals do very little for me, as most of them, including this, require virtually no mathematical reasoning. I do have to admire, however, the ingenuity of the setters and this is a lovely idea of Kea's. Thank goodness for Excel, although there is still a lot of hands-on work to do.