I have one additional suggestion for you that might be helpful to improve the accuracy of the readings.
It just so happens that I am building a curve tracer, and I also want to do some switching of resistors,
while squeezing the most in terms of accuracy out of the setup. (I will post the curve tracer elsewhere in this forum.)
The main idea is to use Kelvin connections! If you take two 4051 analog multiplexers,
you can solder them on top of each other on all pins except for the z pin(!).
So the selection signals are the same, and the y inputs are the same for both chips,
but the multiplexer outputs are kept apart. I assume the resistors to be switched are connected to the y pins.
Then you can connect the current drive (force, in Kelvin terms) to one of the z pins (doesn't matter which one),
and use the other z pin to do the the measurement. That way, the larger current that causes the voltage drop in the switch
is kept separate from the measurement current, which is (should be) very small.
So the voltage drop on the measurement switches should be quite small.
Neat, right?