Impulse and step response of a system that contains cascaded second order sections
Not sure I follow completely, but is this the question: how to obtain the impulse and step response of a filter that is realized as a cascade of second or sections? or are you asking for working code that implements this? 

Your 8th order filter is decomposed into 4 second order sections. The ordering is not really important here. For any input $x$ to your 8th order filter you compute the output $y$ by first filtering the input with the first second order section $SOS_1$ to get output $s_1$, that is $s_1 = SOS_1*x$ (the $*$ means processing the $x$ with a second order difference equation with coefs from $SOS$). Then that output is made input to the second SOS, that is $s_2 = SOS_2*s_1$. So now you have $s_2 = SOS_2*(SOS_1*x)$. And then continue this pattern (output from one section is input to the next) through all sections. 

If you understand this, then it should also be rather obvious that you obtain the impulse response or the step response by letting an impulse or a step waveform be input to your filter and the output $y$ contains the response.

I have not looked at your code. But try to start with a simpler case first just to verify your code and your understanding of the concept.