Where is Sichuan relative to Hubei Province? Communications and transportation being what they were in the 13th Century, it wouldn't be surprising if Gwok Seung didn't learn about the fall of the city until months after the event had occurred. I'm not up on my Chinese geography, but it's possible that Sichuan was already Mongol-occupied territory by the time Gwok Seung traveled through there.
We'll never know if it was Yeung Gor, but consider this: it had to be someone whom the Gwoks could have trusted with such an important task AND had the ability to carry it out. Other than Yeung Gor, who else could it have been?Yang Guo has to miss out the battle as well but get back in time to get the package Guo Jing left him to deliver (assuming its Yang Guo who delievered).
I'm reasonably assured that East Heretic, South Monk, and Central Mischief were all dead before the fall of Seung Yeung. Although they were surprisingly robust at the end of ROCH, all three were, in reality, decidedly old men. Even great martial arts masters can cheat Father Time only so long...It all comes down to slow timing and missing out, and this has to happen to many characters (Guo Xiang, Yang Guo/Xiao LongN, ROCH Greats(if they are still alive by then, possibly not). The missing out will need to happen to these ppl to explain their survival or the lack of their description in the final battle.
Actually, life is like that, more often than not. It's not the most dramatic thing, but it is fairly believable.Rather weak IMO to leave it all to poor timing.