Well, think about how you use your middle G harmonic as a reference point for your hand in lower thumb position. You should comfortably have the G, A, B and C under your thumb, first, second, and third fingers. You can use the D as the same kind of reference point. Spend time slowly working on your hand shape up there, keeping your thumb on D, first on E, second on F# and third on G, or other sets of intervals. Do what's most comfortable at first. Try slowly playing the third octave of a G major scale up and down. Be patient with yourself. Your callous on your thumb will come in time, it will hurt a bit for a little while but doing exercises such as this will only quicken the process. As for your tone, make sure you are moving the contact point of your bow closer to the bridge as you play higher. Be patient, perseverance is key, the results will come. Good luck!
