just flew JX LAX-TPE-LAX on an AS Business award and I was able to check in through the app. Surprisingly, AS transferred over all of my passport info to JX so I did not have to add it manually on the JX app. Selected mobile BP and then took a screenshot of it. I was able to use that screenshot bp to access the OW lounge at LAX after I arrived on my separate tix from SEA. I did head to the gate a few min before boarding began to see if doc-check was required (and it was). On the return from TPE, used the app again. But once I got to airport, I went to one of the check in kiosks in the terminal to get a print out. I think since I scanned my passport in the kiosk to access my reservation, doc check was not required at the gate.

Also, a few notes on flying JX...
1. due to the shell of the J seat and the crappy window spacing of the A350, most seats have awfully misaligned windows, to the point that you do not even have a window view. In my experience, I'd say 5A/K is the best, followed by 4A/K, and then 6A/K. Other rows have the wide frame instead of a window. LAX flights are overnights both ways, so i guess it will not matter to some.

2. JX A350s have dimmable windows...so expect the same thing as flying a 787. The windows do still let some (faint) light in when it's at its darkest setting, just like on the 787-9/10s.

3. in TPE, if flying to LAX (and soon SFO), you will not get access to the JX Galactic lounge since you'll be departing from a different terminal. US flights depart from T2 while JX Asian flights depart from T1 and i do not think they are connected post security. I also do not think they'll let you access T1 with a T2 bp though. Anyway, i did not know this so I was a little let down, especially since T2 lounge JX J pax use is the Oriental Lounge, which felt like a priority pass lounge.