That US will come into direct conflict comes without saying, since no one has the ability to attack China by themselves.
So what you're saying is essentially that US will only attack in Taiwan and around Taiwan, not expand the scope of the war just like Russia didn't attack Ukraine friendly assets directly helping Ukraine from the outside. And in exchange, the idea would be that China would also restrain itself to not start attacking outside Taiwan. It's an interesting idea, but also very hard to say if both extremely opposing governments can really find enough common ground to hold a gentleman's agreement like that.
The thing about the Ukraine war is that it's the Russian military's overwhelming power that lets them restrain Ukraine to mostly being in the Donbass. Ukraine is constantly actually trying to expand the war, it's just mostly not working due to strategic logistics and tactical concerns.
Since US doesn't have that overwhelming military power and even further if they limit themselves in the conflict which exacebrates their lack of airbases compared to China, I don't think the gentleman's agreement will hold.
I mean if you think about it, if Ukraine had more than half of whole Russian navy VLS count but all concentrated in the black sea, had the ability to take out Russian theater airbases 24/7 and has local superiority in airforce numbers etc. I don't think Ukraine would agree to only fighting in the Donbass. Them being limited there currently is more a function of their weakness than the success of the Ukraine model as a "safe" proxy war.
