ShivanAngel wrote:The use the highest only applies to cavalry or mounted models.
This is almost compleetly true.
The multiple characteristics rule applies to units when the test is on a per unit basis (like panic tests, stupidity checks and so on), and it applies to models when the test is on a per model basis, (like tests caused by Pit of Shades, Purple Sun and so on), so he had no basis to use another model's characteristic.
Furthermore the Plague Furnace and Screaming Bell are not classed as warmachines but rather as unique, and the skaven army book says its a special mount, so as per the mount rules the best statistic he could (or to be precise must) use is the highest one on that model, which in this case is the priest's I 5, while for dwellers below for instance it would be the Furnace's own S 5.