You have the F and the FN and both don't provide J11 nor J10 Supercurise capability right? But the Su 35 does have this capability and you need it for J20 to reach the performance of a 5th fighter, just like you need such an advanced PESA radar, since your fighters still only use puls doppler radars, although there are some rumors about own PESA or AESA development, but till now there wasn't any confirmation about AESA in J11B or J10B right? Infact the reports about J10 are not even clear if it's a PESA radar it might get. So no matter how long you deny the fact, Russia is still ahead of you in engine and radar developments, while your own engine development suffer from less maturity, partially overweight or even not offering more thrust than the Russian engines they should replace. These are parts of the reasons why PAF is still waiting for an Chinese engine, that might be good enough to be used in JF 17 and to not be dependent on Russian engine supplies, which is funny, since China itself is highly dependent on Russian engine supplies for nearly every aircraft you have in development isn't it?
But you are right, for Russia selling Su 35 to China would be usual business for them, since they sell stuff to others, that they consider as not high end anymore, especially when they offer us more advanced things.
Btw, Russia loosing India as it's biggest client, really?