Devils Postpile season info can be found here;

http://www.nps.gov/depo/planyourvisit/hours.htm

I've been to the Postpile in mid to late Oct. on several occasions without any problems but that doesn't mean it won't be closed early this year because of an early storm. Oct. is a great time to visit the locations you're considering (Mammoth and Devils Postpile). The days can still be warm but with cold nights. There are definitely no crowds and you can drive down into Postpile.

I can't comment on the rest of your questions, sorry.


hiiker