Sync and async clocks, fitter, ...

1) If you use set_clock_groups to say two clocks are asynchronous, then the fitter doesn't have timing requirements between them, since as you point out, the phase-shift is unknown.

2) The PLL output and oscillator will not be edge aligned, but the difference will not be random, it will be the same compile to compile. Just as importanly, TimeQuest will calculate that difference, use it to drive the fitter to try and meet timing, and report whether it was successful or not.