CI used to do TPE-ANC-JFK, I can't remember everything exactly but I seem to remember:
1 - Before 9/11, passengers cleared immigration at JFK
2 - After 9/11, passengers cleared immigration at ANC
3 - Stopover was allowed at ANC, but only stopover, not separate TPE-ANC ticket
Same thing with BR's TPE-SEA-EWR.
I wonder if CI will allow PHX-LAX stopover traffic.
CI was doing charters to PHX for TSMC. I even saw a bunch of their employees picking up cases of water and bananas at Costco

So, I guess they could start much quicker than JX.
I guess this is Taiwanese competition for you. BR will probably start too. It actually operated a charter KHH-PHX once.