Current measurement in a Buck Converter
Your assumption about duty cycle always increasing as load increases is only true when the converter is in discontinuous mode (i.e. when the inductor current falls to zero every switching cycle). You haven't said if your buck is synchronous or not, and haven't mentioned your inductor size, so we cannot tell when (or if) your converter would transition to CCM from DCM.

A quick-and-dirty way to get a representation of the inductor current is to put an R and C in parallel with the inductor such that the R and C give you a voltage proportional to the inductor current - this voltage can be fed to a comparator to generate an overload trip signal. (It works well in CCM.)