I'm thinking of getting the Asia version physical copy with English language. I'm in USA so the code online won't work for my account.

Will I be able to get an American code online and play the game?

It says the code on PSN is free here

https://store.playstation.com/en-us/product/UP9000-PCSA00152_00-ONL0010000000000

If im reading this correctly you want to buy the physical Asian version, but cant use the online pass that comes with it since you have a US PSN, and try and use the US online pass linked in the OP with the Asian version? Odds are that will not work, in every case i have seen where the base game is from a different region then the DLC it doesn't work. You would probably have to make a PSN from that region if you want to play online with that version.

Is there any reason why you are not just getting the US region version? that looks like it would be the easiest option.

Yep, that wont work.