I'm trying to look at what LE possibly knew and when they knew it.
After Sept. 11th, they knew GP hadn't been heard from by her family since Aug. 27th (or Aug 30th if you count that text). After impounding the van on the 11th, they also knew there were electronic devices still inside.
After the 14th, they had legal access to those devices, specifically GP's external hard drive. They would likely have then had the date she last worked on her computer editing, which maybe did not require wifi or cell reception, like text, calls, or data would. ???
With that possible hard drive information known, 4 days later, on Sept. 18th, the FBI asks for witnesses at Spread Creek between
Aug. 27 - 30th. Cell pings and/or camera captures could have placed them at SC on the 30th, also,
but since GP hadn't even been located by Sept. 18th (or as
@jjonez pointed out, was even known to be dead), I don't know that LE would have gathered camera information yet. Maybe cell records. JS reportedly talked to the FBI on Sept. 16 about seeing the van (and BL), but she said it was gone by the 28th.
But, based on this early date and timing, I think external hard drive info might be very important here.
On the 19th, one day later, they search SC (likely on tips) and find GP.
Two days later, on Sept. 21, the autopsy is done, GP is identified, and COD of homicide is released.
Next day, 22nd, FBI issues arrest warrant for activities beginning Aug. 30th. Credit card info was likely known earlier, though, imo.
GP's mom said in an interview that GP sometimes didn't get great phone reception, so NS said they only talked a few times a week. I have to at least consider, based on what little information we have, that GP was in Spread Creek, out of contact to anyone because she had no reception for cell or email capabilities, but could have been in the van editing a couple days worth of GTNP footage while BL reportedly hiked. I think it's at least worth considering they were taking a short break from each other before heading to Yellowstone.
By all accounts, she could have died the evening of the 29th or even the morning of the 30th. Maybe I just want her to have had a couple extra days on Earth, but I think the information off that hard drive will very much narrow down the day.
All JMO...