Tbh I'm not too worried because unlike JAL, ANA requires its customers to PAY an extra $25 ($90-100 for my whole itinerary) to select a seat EVEN IN VALUE PLUS (which now costs $2700++). That's why you see the window and aisle seats not in the last 7-9 rows blocked. If this was JAL, I would've cried because I know my chance of getting a window will be slim because JAL's basic economy still allows a free seat selection, but not if booked from American.

I checked for my flight, those free window, middle, and aisle seats at the back of the plane are now mostly full; the paid windows and aisles, however, aren't. I think your best chance would be to call ANA, register all of your info (because United won't do it for you and without it, ANA cannot check you in online), then check-in at T-24. That's how I stole 3 out of the last 4 window seats on my last JAL flight when I traveled with my companions. If not, stand in front of the line at T-3 at the airport; chances are you'll get the second-to-last or the last window seat available.

At the end of the day, idk if ANA plans to switch last minute from a 787-9 to a -8 or a -10 or even a 777-300ER for SEA; so I might just hold off and pray.

EDIT: Just checked the seat map for my itinerary, and I kid you not; on the SEA-HND leg, there are more empty aisle and window seats compared to middle! For the HND-SIN vv leg, it's still a tied gamble.