Huge lead? I don't think so!
1. What about the An-70? If it is not interfered with (production delayed, funding cut, inferior financing offered etc.) it should take a decent slice of that market. Similarly, for military operators with relatively deep pockets (say, Algeria) the A400M is very much a realistic option - and the Il-476 is no match for either of them.
2. There is NO reliable info on China's engine development schedule, so all of your above considerations could turn out to be founded on hot air.
Nonetheless, yes - propulsion will likely remain a disadvantage for some time, but as I said the Y-20 will probably be able to give competitive performance even with somewhat inferior engines.
It's a procurement blunder on the scale of the Nimrod MRA4 and similar Western mistakes - only you don't hear about it because it's Russian, so the Western press is neither interested nor competent.