I’ve heard of people finishing in late October, but it’s pretty rare to comfortably do that. The latest anyone finishes regularly is probably mid October. Any later than that and you run serious risks of being snowed off the trail and unable to finish or dying of hypothermia. And to be honest, I would plan to finish before the end of September if at all possible. I and some other thru-hikers rain trail magic at Hart’s Pass (northernmost road in the US on the PCT) this year on September 26th and at night the temperatures were well below freezing at night and not comfortable during the day. Thankfully it was sunny.
At any rate, there’s no need for you to go to the wedding from Donner Pass if you don’t need to go back until the end of July. You should consider other locations. By the last week in July in both 2008 and 2009 I was in Ashland, OR which has great public transit options (train, bus, even a small airport, although I don’t know what you could fly out of there. But transport to Medford, which has a bigger airport). If you’re not confident you could make it to Ashland, you could always hitch out from other towns. Etna has a bus called “the stage” that will connect to other towns that have longer-distance busses that’ll get you somewhere with an airport or train station. Hell, Dunsmuir (A 5 mile hitch or so from Castella) has an Amtrak station that lots of section hikers use to get on the trail.
So weigh your options. I’d say by July 20th you could easily be in Dunsmuir or Etna. Then when you come back July 31st, your positioning will be much better for a late September finish instead of a risky mid/late October finish.
Joker