Answer 1

Change line 30 from:
return func_name(x,y)
to:
return globals()[func_name](x, y)

For a local function:
locals()[func_name](x, y)

If this was in a module (class package) you might be able to use:
getattr(self, func_name)(x, y) # self for the current module otherwise use the name of the imported module or object

You can also change your function name list to:

avail_functions = [
add_me.__name__,
multiply_me.__name__,
substract_me.__name__,
divide_me.__name__,
]
using the __name__ magic method on the the function will return the name of the method as a string.

Answer 2

Thank you very much!
I used "return globals()[func_name](x, y)" instruction and it works!