1)How large are the known lands of Tuwa? Does the map represent an area the size of France, or Texas, or the Russian Steppes?
Don't know, exactly. I based Tuwa on both the Russian Steppe and the Great Plains of the US, prior to white men's arrival. So they're big and broad with lots of spaces to fill in.
2)The actions of the Keepers have helped to advance the Great Glacier, but was it always there before? Or was there a larger expanse of water or land that's since been hidden. Did anyone cross the whatever-was-there-before in stories of old, and return?
The Great Glacier has grown exponentially with the arrival of the Keepers - it is in fact their harbinger. I would say the Free Peoples do not know what's on the other side.
3)The Children of the Dawn are referred to collectively, but there are several mentions of the Free Tribes. Are these several distinct "political" entities, or just loose collections based on familial ties? And how populous is Tuwa?
The Free Tribes are the Tribe of the Hills, Forest, etc. etc. - all Free Peoples and Children of the Dawn. The Tribes in this case are races, who share a spiritual and genetic ancestry. I wouldn't really call them a political set of groups - that would reside in individual tribes/communities, some of which might be organized along spiritual and genetic lines.
Tuwa is not terribly populous. The Keepers are doing their best to see to that.
4)Should the elha be hated and despised, as they are too far along the road to ruin; or can they be return to the ways of the Free Tribes?
Elha are usually despised as cowards and sell-outs, but unless they are also Taken, they are not beyond spiritual redemption. Elha are more like "sheeple" who didn't have the will or strength of character to stand up for the Old Ways, rather than "bad guys." Though many are also informants and collaborators.
5)Lastly, I have a thought to have the lands outside of Tuwa populated with other cultures, either not affected by the Ghula, or starting to get there own version of it. Is it possible to integrate these outsiders into the longer story arc, or will that detract from the theme of Tuwa too much?
That's an interesting idea, and I'd say go for it! FWIW, if I were to do something like that I would make sure this new region is geographically very different - the Free Peoples are nomads and have followed the Steppe as far as they can for centuries. Maybe having some people come from across the Giant's Teeth, like a combined "new tribe" of Saurians and Drakes or something like that could make some interesting new challenges for the Heroes or mix up the somewhat one-sided conflict in Tuwa...