Kragar refers to Fornia as being Sethra's "neighbor":
I think the "Barnsnake River" is almost certainly an alternate name for the Yendi River. Here's what this area looks like on Steve's Map:
Dragon map
His claim that the Barnsnake river is two-thirds of the way across the map doesn't seem to line up with Steve's map, where the Yendi is almost directly north of Dzur Mountain.
Also, while he only mentions seeing the foothills of the Eastern Mountains on the map, he laters claims to remember one of the mountains from (presumably this same) map:
Based on South Mountain being the first peak you see when approaching from the southwest:
I think Mount Drift and South Mountain are one and the same.
How far do these foothills extend? No further than the Laughing River to the west, the Shallow Sea to the south, and Barleytown to the southwest:
I concur with
Alexx's assessment that Barleytown is about 270 miles from South Mountain.
The closest the Shallow Sea gets to the peak of South Mountain is about 325 miles, so I guessed that the 270 miles is from the base of South Mountain, not the peak. I think Barleytown is probably also a stopoff of the road that runs east of Dzur Mountain that multiple journeys appear to take place on.
I added foothills extending about halfway to these three bounding points:
Barleytown and the foothills of the Eastern Mountains
In Release 2 of the Map of Dragaera, I thought Sethra's Army marched over 300 miles a day in the early part of their march (based on Vlad's claim that he'll see the Eastern Mountains after two days of marching). The distance between Castle Black and South Mountain (the closest Eastern Mountain) is actually greater on Steve's Map, so I at first thought this march was even more incredible (closer to 500 miles a day).
However, in a subsequent rereading of the book, I realized that I overlooked something; while Vlad is told he'll see the Eastern Mountains after only two days:
It's actually more than that before he sees them. Shortly after the above exchange, they camp for the night, and Vlad's told a little about the plan for the next day's battle:
That night, he does some light sabotage, and then the next morning, he fights his first battle, after which:
I count six days (one of which they appeared to not move, so five days of marching) in total before he sees the mountains.
From how far can you see a mountain?
This depends primarily on two things: the height of the mountain and the curvature of the planet (assuming viewing conditions like the weather are ideal).
Steve already told me the planet was "roughly" the same size as the Earth, so the only question is South Mountain's elevation.
About all that's said about it is that it's large. To determine what "large" means, I need to know what "average" means, and I was unable to find anything like an "average height of mountains" on Earth for the simple reason that there is no single agreed upon definition of what a mountain is.
I went through
this wikipedia page that lists the tallest mountain in 84 different mountain ranges and found that they ranged from 9,281 feet (Mount Fogo in the Fogo mountain range) to 29,029 feet (Mount Everest in the Himalayas). I imagine most of these would be described as large (at least within their own range), so I decided to use their median height (about 13,000 feet) as an "average large mountain height."
That means in five days, Vlad traveled all but 140 miles of the 1,268 miles between Castle Black and South Mountain, which works out to be about 225 miles a day, which is still a lot, but Paarfi would not have been impressed:
Back to Vlad's march in Dragon:
Vlad appears to travel north of South Mountain, but then heads back south and the battle ultimately takes place near the Wall and Baritt's Tomb:
There are other references to Baritt's Tomb that put it pretty far north:
But Morrolan assures Vlad that the Tomb is nowhere near Deathgate Falls:
Vlad estimates the battle takes place about 300 miles north of the Shallow Sea (he says Khaavren's Sea, but I think that's another name for the Shallow Sea--regardless, Steve's Map makes it clear there is only one Sea near here):
I think the Wall is to the southeast of South Mountain, since Vlad says it's to the northeast of the battle location. Since South Mountain is 300 miles from the Shallow Sea, I think the Wall is very near to it (the further south I put it, the more wrong Vlad's estimation of the distance becomes). I put it about 50 miles to the south of South Mountain based on my theory of mountain sizes.
The Flatstone River
The spinoff Vlad mentions is probably the same river Mica mentions:
Vlad thinks the river branches off of the Laughing River, whereas Mica thinks it's formed by streams coming off the mountain; rivers don't normally split, so I think Mica is probably right.
I think we're even told the name of this little runoff. Earlier, when Sethra is planning the campaign, she mentions that they will end up in the vicinity of the Flatstone River:
I drew the Flatstone River starting north of South Mountain, flowing to the west near to the Lauhing River, then curving back to the south and hugging the Eastern Mountains to the Wall (where Vlad mentions it), and from there, straight south into the Shallow Sea.