PLAAF FC-31 isn't worthwhile, for the simple reason that the FC-31 does not fit a role. People speculate about the FC-31 forming a low end of a PLAAF hi-low mix, but the FC-31 is not a true "low" fighter a la the J-10, it's a twin-engined aircraft using mediumweight engines as opposed to the heavyweight engines of the J-20. As such, it's likely not to see significant cost savings compared to the J-20, while sacrificing capability such as sensor and physical ranges. The only advantage might be enhanced radar stealth, but the J-20 is already likely to reach -40 dBsm (J-20 in least favorable RCS studies is -20 dBsm minimum, so's the F-22) with existing metamaterials in targeting radar ranges.
It is more suited for carrier aviation if it does see service, since SAC has experience with carrier aviation than Chengdu lacks.
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As for variants, the Chinese and Soviet attitude toward variants is different than the American attitude toward variants. What went really wrong with the F-35 is that they tried to get all three variants operational at the same time, when what they should have done should have been to get one variant operational, then build the others as modifications. The variant philosophy of the Chinese, Soviets, and even the Americans (see F-15E Strike Eagle, EF-18 Growler) has been successful. The F-35 is not due to a single, obvious mistake.
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As for JH-XX, the advantages of the JH-XX has to do with its airframe size and class. It's purportedly 30 meters long, which puts it as significantly larger than the J-20, meaning that it can carry more capable avionics, such as longer-ranged EODAS or AESA, perhaps even counterstealth radars. Unlike the H-20 platform, it's supersonic capable, so it can keep up with J-20s supercruising.
If the PLAAF DOES go with significant numbers of FC-31s, what you'd see would be FC-31s escorting KJ-300s or 600s in a counterstealth role, given that neither wish to be fast, while JH-XXs would escort J-20s to provide counterstealth detection.
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A final retort you could make is that the Soviets ran a combination of Su-27s and MiG-29s, with the latter being a fighter using two medium-weight engines. However, the MiG-29s were designed for very short range and originally complemented the MiG-25s and MiG-31s, who were very long-ranged and BVR strong. Likewise, the MiG-29 never saw the same development the Su-27 did, where the Su-27 was developed into a family of fighters, and Russian aviation inventories are more tilted toward the Su-27 derivatives; only 255 MiG-29 and derivatives are in Russian service, while they maintain 229 Su-27s, 111 Su-30s, 124 Su-34s, and 98 Su-35s, for a total of 562 Flankers and derivatives vs 255.
You could see the J-31 as a skeet fighter vs the F-35, boasting a possibly more capable radar (AFAIK the J-31 does not have a larger radar aperture than the F-35) to present advantages, but its inability to go beyond the immediate Chinese vicinity (Taiwan, South Korea) means that J-20s are still a better option for what China is trying to do.