A critically wounded soldier who recovers gets a permanent Will penalty due to the psychological trauma. Does this stack for each time the soldier has to recover? If so, what's the limit on how low a soldier's Will can go? Does the penalty eventually stop stacking? Do low-Will soldiers always die instead of getting critically wounded? Or can I eventually deploy negative-Will panicky messes?
There are no caps on how low Will can fall, except that it never goes negative.
It might be possible for it to fall to 0, but that would take some doing. By the time your soldier's will gets low enough that you could hit 0, the soldier will be panicing at the drop of a hat. Any time a window breaks, they would panic.
By that point, your soldier will be effectively unusable in missions.