and there is no absolute standard earth other than some arbitrary less than a higher potential.
There is, it is the potential of a stake in the ground. A lot of people mix up this with the common signal point in a circuit as these are often tied together. I think you are missing the point about the accuracy you need on those potential dividers. There is a concept called Monoticity basically it means that a higher reading is given by higher voltages. A not altogether unreasonable thing to ask from an A/D.
With your scheme you need a very high order of accuracy on your resistors so that the readings made on one range map to a reading on the range next to it. With only 1% resistors you can't do that. That is why a good auto-ranging multimeter is quite expensive or rubbish.