WiFi signal strength distibution
I wonder how wrong is it to assume that in general case for a non-directional antenna the wifi signal level distirubiton can be described with the function quadratic loss


Pretty wrong. 

WiFi is an indoors technology, and your model would apply to free-space only.

You're not taking into account any small-scale fading. That won't do.


  I need all this for my graduate project 


Then you'll need to do quite a bit more research. 


  in which I need to create a model which would provide the wifi signal level in desired points in a room after taking some measurements.


What you want is typically done with ray-tracers and/or finite element simulations.