It’s not about fighting, it’s about deterrence. China has the clout to force Trump to pause, if Trump knows it’s Chinese fighters and ISR planes manned by Chinese pilots and crew in the air. Besides, China would be restraining Iran and that would be to Trump’s favor.
Now Iran could wager that their nuclear program would be destroyed but their regime would survive. It seems like they don’t want to stop at the program, Israel wants to drag the us into causing regime change. So the Iranians have to figure out expensive Chinese help is better or risking their regime’s survival. China is probably the best country for them to make this deal with, as China would not interfere in their internal affairs and only stick to their strict bargain, not very different to the gulf monarchies and their protection deal with the US.
The PLAAF probably wouldn’t go to fight foreign air forces in a sector of Iranian air space, but once foreign aircraft leave that air space, they would move in and maintain CAPs to secure it, creating a situation where foreign air forces would risk having to fight Chinese aircraft if they returned.
The PLAAF and PLAN would get real world experience as a high end peacekeeping force.