Complete this plane and export it at prices and to nations that allow leveraging of political gains. How likely is PLAN to find favour in this over J-20? They need and have the funds for longer legs. J-20 and J-15 combo is far better. We shouldn't have LO platforms with shorter legs than the rest of the aerial assets. PLAAF are not stupid enough to drain their funds into two stealth fighters. China is not yet the US equivalent in defence spending and won't be for quite a while. Better to use those funds in acquiring more J-20s and upgrading that platform and supplementing the rest with more J-10s, J-11s, J-16 (Growler types), and drones.
A few issues I'd like to address:
1. How do we know what the PLAN's range requirement is? If a slightly enlarged FC-31 could give its current 1250-km radius a boost to 1500 km or more, so who is to say that it would still be insufficient for the PLAN's needs?
2. The PLAN would need to balance range, cost, and also the # of fighters each carrier could embark. Obviously, the main advantage that the FC-31 brings would be its small size, which could provide the PLAN with benefits that may or may not outweigh the J-20's range advantage. Furthermore, I'm not sure why you are adamant that LO fighters match the range of other aerial platforms; this certainly isn't the case with the Su-33/MiG-29K combo or the F/A-18 & F-35 (or F-14 for nostalgic people here).
3. Having two LO fighters is exactly what I expect the PLAAF to do. The J-20 could replace twinjet air superiority platforms such as the J-11, while another would be needed to (1) replace single-engine fighters like the J-10 and (2) provide a cheaper LO option to be fielded in greater numbers than the J-20. I'm not saying that the other jet must be the FC-31, but a high-low combination among 5th generation platforms should be expected of any major air force with multiple platforms spanning several niches.