I didn't actually bend anything, I initially grinded (ground?) mine thinner.
The thermostat controls the on/off cycle of the oven based on its own (the thermostat'ss) temperature. Oven getting 'too' hot -> turn heating elements off. Oven has cooled down again -> turn heating elements back on. It's a breaker for the electrical circuit. That's why you see the heating elements cycle. For pizza baking you want it to be on.
Anyway, the little screw and white pin go inside the screw on top of the thermostat. The small screw is supposed to push the white pin down thereby pre-bending one of the contacts below causing the switch to 'off' position to occur at a later point (when the thermostat is hotter).
You probably adjusted it too heavily so it fell out.
The thermostat is just an/off switch and works with a bi-metal. Once it reaches a certain temp it bends so that the oven does not get any more electric contact.
Maybe the two attached imges help.
Your plan with the two thermostats is likely not going to work - the thermostats don't measure the temperature of the heating elements but the temp right where they are located, so if you put both in the same place then they will more or less turn off at the same time, too. It's a pretty basic/simple/stupid setup the way it's designed.
Depending on how your device is wired (for example both heating elements in series) they will both shut off at the same time anyway, no matter what.
IMHO, a second heating element on the bottom is not needed. The bottom heat is plenty, just top heat needs more power. In the end you will likely need to find a different solution than the thermostats, though. What I did is electrically not safe so I won't detail it. Let's just say that my oven has no more thermostat. That comes with a price, though - you will constantly need to monitor the oven or it will melt (at least the plastic and the wiring). A big fire hazard! You will also need high heat resistant wiring if you want to use double heating elements (and a whole shebang of other parts).
As for a possible setup: put one (or if you insist two) V-type heating elements in the bottom and the circle ones in the top. For neapolitan pizza you want to direct most top heat toward the cornicione. The round ones are better for that. For the bottom, the V-type are better because they produce a more even heat distribution. You will still probably want to use at least the reflector because it redirects the heat behind the heating elements towards the pizza instead of warming up the device case even further. I personally find no use for the deflector because my oven is plenty hot and I see no burning cheese in the center of my pies.