The F15A used poor twin engines, but the F15E/F used upgarded ones, and F15E/F is still the best fighter in the world. Which proves that the plane is an outcome of balancing all the factors, including enigne, electronics, materials, aerodynamics. It's obsurd to judge the plane only by engines.
Plus, except F35, all the past US naval's fighters are two enignes.
The PLAN didn't ask J31 to be capable of the "V" from V/STOL. The combat envrionment nearby China dosen't asked for it, currently the F35B is no sense to PLAN.
Plus, i have to repeat again, do not use your hobby to challenge professionals. DFI is good place for exchanging info, but not the right place for really going to the technical details.
You would show off your out dated knowlegde in the title of engineer, but as a former computer sicence developer, i never show off analysis on how F22/J20 fly-by-wire system.
Sometimes, i guess if i throw you a DJI Phontom4 SDK, can you make any reasonable change based on such mature civil drone platform. If not, will be laugable to talk about how stealth plane works...