Voltage measurement circuit
I see what you did there with using R2 as a shunt resistor. The reason why it is not working is with your maths or your resistor values. I'm not familiar with the math along with using shunts with such high resistances but it is best to keep R2 as low resistance as possible like 0.1 ohms (yes that small) or less. If your multimeter readings don't go to such small values then you maybe able to get away with 10 ohms. Then you can do the maths you suggested from there. Basically your concept is correct.

Also your voltmeter will affect very slightly to the circuit in the manner of an additional resistor in parallel but it's so small that it shouldn't matter. (It has a resistance of like 10Megaohms generally). If your material has resistances around that region then it's probably going to affect the readings.