You asked a question which had never ever crossed my mind. No immigration anywhere in the world cares which airline you flew in on or which one you have tickets to flight out on.

Check-in: if by chance (very slight chance), anyone asks you for proof of how to depart China, all you need is a ticket showing you will leave. I personally have never been asked for this in 30 years of flying to Taiwan and China.

And when you go through China Immigrations (not Customs -- that's for your bags), almost never do they ask for proof of a ticket to leave. Again, I have never once been asked this.

The last thing they would hassle you about is flying out on a different airline than you flew in on. That's not how things work.