Hello !
You may connect the IN pins of the unused AOP's to avoid electrostatic shock (but not the OUT pins !).
Then I cannot figure out how the AOP's are wired, maybe you can use this kind of circuit :
-electronique.aop.free.fr
-electronique.aop.free.fr
Or these (I think you used these) :
-electronique.aop.free.fr
In those last ones, you can modify the gain without modifying the cutoff frequency
For the limitation you should use diode like this (D1 and D2):
Sorry for the french links :~