For a 2-hour flight, at that kind of price premium, there is no way I'd pay for F over Y on CA. The value just isn't there, IMO, and in case of IRROPS, I wouldn't bet that a passenger on F status would get that much more relief than a Y passenger. After all, it's Air China. Assuming you have a foreign face and a big complaining mouth, that might get you just as far in a free-for-all scramble for alternate seats on another one of the sparse flights, should it come to that.

As for cancellations and delays, etc, yes they happen. MIAT has a better reputation than Air China on this issue/route, so you might consider MIAT if prices are nearly the same for Y. Air China will cancel if loads are too light, and use some cockamamie excuse, count on it. And as you've noted, there aren't any other options except via Seoul on Korean. Or the train PEK-ULN, which this time of year (summer) gets booked out pretty solidly.

If I HAD to be in ULN on a certain day, I'd be giving myself some breathing room and go up at least one day earlier, in case of IRROPs or other snafu. And I'd probably choose MIAT.