Don't have Engine Manual in front of me, but your exceedance is well within limitations if I recall correctly. It's such a miniscule exceedance it's tough to assess if any damage was done, although I suspect not.
White smoke is not unusual, It's indicative of unburnt fuel. Usually whatever is left in the nozzles that didn't get purged during shut-down.
23.9 Volts is plenty voltage. Many have started a PT-6 with much less, in tougher conditions.
While you should ideally get your quick-turns to 10-15 minutes, I've done plenty a runs where starts and shut-downs were done in very quick succession (rigging/balancing adjustments).
Good PT6 technique with a warm engine is don't look for a specific Ng value to introduce fuel. Keep the thing motoring until you start to see the Ng begin to stabilize. My rule of thumb is once it takes more than a second for Ng to increase 1%, time to introduce fuel. Some will state to wait for fuel introduction until Ng has stabilized.
While what has occurred isn't the best for your engine, with the excellent construction and materials behind the -42s I think you are safe.