I don't have an exact answer to this... But, for example, OHSU sent an email out that said "INBDE results must be received by ADEA PASS by January -7, 2022." Not sure how other schools are handling it, but I hope this helps. I would just call and ask each and every school I was applying to if I were in your situation.
Some schools like UIC (for example) have on their websites that they encourage early application... Other schools like OSU (for example) write on their website that no apps are looked at until after the deadline so take your time. the PASS FAQ page says to submit the app before all of your letters are in and your schools will see when the last ones are received.
Some schools in certain programs ask you to write at the end of your PS why you want to go to their school specifically... It's not common though. Also, some students like to tailor their PSs to each school regardless of what the school is looking for which sounds like a TON of work. All the schools I applied to are getting the exact same PS.
Background is up to you. My background is a beige color because I didn't have any white walls with nice lighting available. I think when they say "passport/2x2-style photo" they just mean, we want to see your head and face close up looking straight and less about having a perfectly white background. These photos are for them to spread out pictures on a table and sort their rank list. Don't take my word for it, but I don't think it'll be an issue.
WOOOOO! That's the higher end of what you were hoping for. Congrats! (also, I think they will only help you since you are above the national mean)