Why no Holy Spirit during Requiescat?

Holy Spirit consumes Requiescat stacks, as do the Confiteor combo actions. The Confiteor combo actions are barely worth using with no Requiescat buff

Why is hardcast Holy Spirit not really 350 potency?

PLD auto-attacks are normalized to 90p every 3s, so with a 1 second cast time (1.5s tooltip cast time minus 0.5s slidecast window), auto-attacks are delayed by 1 second every hardcast. On average, hardcast Holy Spirit is equivalent to 320 potency.

What GCD speed should I use?

2.50 is ideal (see the Best-in-Slot page)

How do the 6.4 changes affect the rotation?

You can now also hold Atonements for Fight or Flight like we could do with Holy Spirit before, making Fight or Flight windows that previously have Fast Blade or Riot Blade stronger. Simply begin a new Royal Authority combo before spending any Atonements/Holy Spirit, but make sure to spend them before finishing the combo.