I posted in another thread about Best Buy. First, this year it seems (at least based on what I've read) that they aren't doing in store pickups, it has to be shipped. I assume this is due to the mess from years past where they haven't had the stock to fill those pickup orders. I've also read (as recently as last year I believe) that a large number of shipments did not arrive to customers on time (Best Buy offered gift cards for this error).
I was going to order from Best Buy this morning, then reminded myself of this -- I did order a Series 4 Apple Watch last year, and did get it on the promised day (same day it was available and delivered everywhere else), so that was a positive as least -- I don't know that I'd risk it if you're expecting to get it day 1, as again their track record for iPhone launches hasn't been great.
As for your unlocked question, I couldn't say -- I have Verizon, and to my knowledge as far as AT&T goes if you pay full price I believe it comes unlocked.
For the Verizon version at least, they stopped doing it, maybe a year ago, not sure on the other carriers however.A few years ago they also changed $50 for an unlocked phone if you were just paying a one time fee. Not sure if they changed this or not.
EDIT: here is a link