Sorry I haven’t posted before now on this.
If I were setting this up at my house I would put the GE first and the oil second. You are making the change for improved efficiency. The heat pump is most efficient when the differential temperature is the greatest. Check the performance curves; the EF is highest when the inlet water is coldest. When you try adding heat from the refrigerant cycle (heat pump) to water that is already not cold, efficiency drops. Now as to the electric elements. Both the compressor and the elements operate on 240. There is a switching mechanism that stops the compressor and transfers power to the elements at a certain temperature. I would get a schematic from GE and figure out out to route that switched power right back to the compressor since you would have oil heat to finish the job. This would then operate the GE in its most efficient range and the oil would only come on if necessary but would come on to prevent cold showers. And the elements would never come on. GE might even have someone in tech support that could direct you on this.
Applied Energy Recovery Systems made a stand alone heat pump for making hot water. AERS was acquired by AO Smith.