Aye, we finally figured out what was going on with the exta damage, and that is was not, in fact, related to triggering PV.
However, as stated, PV only procs for Gust/Barrage. Neither Tympest or I could get it to fire off for Bolt, Blast, or Tempest. That said, having it work for only 2 out of 5 attack spells is disheartening, especially for the repeater, which does more damage over time than the AoEs to individual targets.
It is also not clear which target(s) get PV procced on them. Is it random, or does it pick a specific one? Is it more than one? Sometimes, I can see it proc up to 3 times, even though I only have two Burning Embers socketed.
Still more questions...