Extended warranties are pretty very frequently through allstate nowadays. I think you're going to end up dealing with Costco for issues for the first 30 days, then it will go to the manufacturer for a year for parts and labor then you'll be dealing with costco/allstate.
I don't think you're going to see much difference dealing with an extended warranty provider wherever you buy the TV... For the times that you need to deal with the retailer, I would rather deal with Costco than most others.
As for bright white scenes, OLED isn't going to be as bright as QD-OLED or other QD-LED technologies, but in a reasonably light controlled room, it should be fine. I can't speak to that specifically. Most of my usage is movies where I really enjoy how dark the blacks can get and how little fringing there is around bright objects on dark backgrounds.