Operational Amplificator Association effect on Slew Rate
The first AmpOp (IC1A) is to isolate the input from the circuit from the protection zeners.


Isolating the input from the protection zeners defeats the purpose of the protection zeners. IC1A is not going to produce an output outside its power rails, which are the same as the rails of IC1D, so the zeners are not doing anything to protect IC1D. 

If IN is outside the rails it will (potentially) fry IC1A and break the circuit, so the zeners where you placed them provide no protection against that scenario, either.


  The third AmpOp (IC1B) is to sum the input with 2.85 provided by the tension divider.


If you want this to work, you need to add a series resistance between the output of IC1D and the input of IC1B.


  But I got concerned about how the slew rate would be affected by associating so many AmpOps.


Slew rate will mainly affect whichever op-amp has the largest output amplitude. 

If IC1D isn't slew rate limited, then IC1A won't be either. 

Since IC1B has gain of 1, it will only slew rate limit if IC1D does too.

Possibly the distortion will be slightly worse from chaining several op-amps. The easy solution is either to find an op-amp with higher slew rate capability.