Yes, we can calculate the phase fraction of multiphase material by XRD.

There are certain factors which decide the intensity of XRD peaks, one of them is the interaction volume. As different phase have different interaction volumes (can also have same in some cases) with the incoming x-ray. After considering all these factors, the ratio of the area under the peak (integrated intensity) will give you the ratio of the phases.

Yes we can.

 I would use an internal standard approach (e.g. corundum) and a Reference Intensity Ratio (RIR) method. Rietveld refinement is for sure a fantastic way to determine phase quantification, but, in my opinion, it is not user friendly as a RIR method. For a better explanation I am attaching a brief presentation of ICCD.