IV relation changing load resistor of a solar cell
You assumptions on the impedance of the PV are incorrect but your understanding of the curve for P=VI is correct.  

The PV is not a voltage source with a fixed Ri. From a  short circuit it behaves like a constant current source( modulated by solar intensity) and from no load it behaves like a lossy voltage source with Ri.

MPT occurs where the the slope of P=0 or dP/dV=0 or dP/dI=0 where the ESR is the slope of the VI curve, ESR=ΔV/ΔI

Can you see how VI curve slope is actually source impedance which changes with V.

Do you think there is any relationship between Voc/Isc=Ra  ( some value Ra) and ESR at MPT?