First, I think Yali is clearly brought on board by Ketheric, I think the Forge is new, or possibily the Adamantine part of it is, I think there are some references to Ketheric or Yali being interested in recreating or refurbishing the old forge, so it doesn't necessarily have to be 800 years old. The book also references Grymforge as being a city, so the city could be 800 years old and the forge could be from Ketheric/Silouv's time.
There are under-gnomes in the Underdark, they don't need to be gnomes from that expedition. I'm thinking these Ironblood gnomes have been mining there for a month or more, were enslaved by the Duergar to help excavate the temple, maybe a week or two ago, with Thulla escaping very recently.
I don't think creating a exacting timeline is on the table, judging from the way distances and such are dealt with, like the ladder down to the Selunite outpost, one of the few travel cutscenes we get (the other also being Underdark related). How long are we supposed to understand it takes to climb down to the Underdark? I was under the assumption that it could take days or weeks of serious spelunking before reaching it, but as you know I'm no expert. So it's just going to be one of those hand waving things, how long does it take? Whatever amount of time makes it work. How far are the distances between A - B and A - B - C? Whatever makes sense for the story.
I'm not too bothered by this mode of storytelling as long as the game doesn't create too many loopholes and internal contradictions.
By design, none of this is clear, and as we've seen a few times, it's subject to change.
