Essayons, I think you've got it right. The way I interpret it, is that if you are shooting at a ridden monster, then per the shooting rules you randomize to see where the shots go. If you use a template, then you do as has been discussed before. And with direct damage spells, such as The Dwellers Below, it says every model in the unit. So, I would interpret as you do, that both the rider and the monster take a test on their own characteristic. You just have to hope they both pass.