The Chinese have experience with canards and are comfortable with the canard concept. If you read the Dr. Song paper, the thing about the selected airframe is that it allows the J-20 to retain reasonable maneuverability even if the engines ended up being underpowered. So it's more about an accommodation to China's technological ability than about pure stealth performance.
As far as not coming up with TVC, the TVC, as well as the WS-15 engines, are currently vaporware. We hear rumors about it, or hear about what a nice idea it would be to have TVC or a 150-180kn engine, but we don't have proof that they exist or are ready to be installed on production fighters.
Everything on the J-20 project is developmental, and given how much difficulty the Chinese have traditionally had with their aircraft programs, one should err on the side of caution and expect that the J-20 will go up in its current configuration.
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One could say that the J-20 project design is the exact opposite of the F-35 project; the F-35 project was highly ambitious, but had no failsafes if any of the milestones were missed. The J-20, on the other hand, can go up in the air right now, with a PESA radar, with 132kn WS-10s / AL-31FNs, and minimally-tested weapons bays, and still be a decent and capable fighter with none of the developmental technologies perfected. It, however, would be much better if it were to get AESA, if it were to get 150-180kn WS-15 engines, if it were to get reliable TVC, or even better yet, get TVC reliable enough that the tailfins could be omitted altogether. But it doesn't need any of these to be worthwhile.
