The MOSFET driver sheild has some problems in it. Both 1.0 and 1.1 don't keep the voltage of the gate below the Vgs limit, which is usually +/-20V. It also doesn't pull the voltage source from before the switched load, so if the load is resistive, then we get an unknown voltage into the gate. So version 1.2 of the shield will have some fixes.
But how this is supposed to be fixed was a bit of a dark art. I am not much of an analog electronics person, I understand digital fairly well, but not the analog behaviour or idioms much at all. Another problem is, short of building it and watching it explode, I really would prefer to simulate what the behaviour actually was.
The way this is usually done is with a set of tools known as SPICE. Unfortunately, SPICE does not have a very easy to use interface, you can't just drop common components down and away you go. There are some tools which attempt to make this easier, but none are really that easy to drive either.
CircuitLab provides a SPICE-like capability with easy drag and drop design, and it's completely online. With it I was able to correct the design of the board and get the result I wanted and validate it over a wide range of inputs. (It turns out that, as I had suspected, if I limit the current going into the gate, and provide a 10V zener diode between the gate and ground it will hold the gate at 10V when selected.)
Next time you want to do some analog electronics modelling to solve a problem? Try out CircuitLab!