I think it's a 757, not a 777. I also don't think they'd need a crew rest seat on a 4 hour flight.

Usually, if they offering paid upgrades on the website, you should be fine. Blocked seats don't necessarily mean oversell. Flight still shows J1C1D1Z1P1, so they are willing to sell at least 1 more discounted F seat, which you can currently purchase as a one way for $387. I very much doubt they'd be willing to deal with an oversell in F during a holiday travel week for an extra $387 in revenue, but maybe.