How can I simulate limits of active components?
You can add an ideal voltage source in series with the gate terminal. Changing the voltage on the source will roughly simulate changes in the threshold voltage of the MOSFET.

Most SPICE models are intended to fit the typical behavior of a device, not the full range of parameters nor any failure modes. The model parameters are often determined by optimizing a "best fit" between the model equations and the actual device rather than calculating the parameters from device physics. Depending on the complexity of the MOS model there may be a number of parameters that determine the effective threshold voltage so you can't just pick one parameter and sweep it.