Ok, got one more strong argument for (3) observation i.e slight deformity at higher i-v values. In this paper:
M. D. Pickett, D. B. Strukov, J. L. Borghetti, J. J. Yang, G. S. Snider, D. R. Stewart, and R. S. Williams, “Switching Dynamics in Titanium Dioxide Memristive Devices,” J. Appl. Phys...