1. Yes, I think that would be one way to do it. More specifically, i think I would calculate both structures at a range of pressures, and see when the high-pressure phase becomes lower in energy than the low-pressure phase. If you have a literature reference, I can have a look at it, and help you to sue the same method with ATK and VNL.
2. I would probably use PBE or PBEsol for this - I think LDA could give problems for the high-pressure phase.