Past Wilder, they appear to cross the Laughing River twice more. Since they're east of the river at Wilder, that would leave them on the east side after the 2nd crossing. Their ultimate destination is Deathgate Falls, which is marked on Steve's Map, and is west (and north) of the Laughing River. In the full passage I'm quoting from, Sethra switches maps between these two crossings, and I'm going to guess that it's the same crossing, just pointed out on two separate maps.
As to where this crossing takes place, Pel supplies a clue:
Since we know where Mount Bli'aard is, this final crossing must be near the northernmost extent of the Laughing River.
Sethra makes the journey after this crossing sounds like a straight shot, which suggests the Greymist Valley runs north/south. However, on Steve's Map, the southern tip of the Spearhead Channel curves back to the west.
Since the Greymist Valley is 20 to 25 miles long, the Spearhead Channel is actually closer to the Falls than the start of the Valley.
The Valley is described as being half a league high (1.5 miles, or about 8,000 feet):
The above limits where the Ash Mountains can be in three ways:
The Valley is 20-25 miles long, with the Falls at one end
The Ash Mountains start 20 miles from the Laughing River
The Spearhead Channel limits where the mountains can be
Ash Mountain Limitations
Paarfi refers to the Blood River (which runs down the middle of the Greymist Valley) as having a west bank:
This establishes that the river runs north/south to at least some degree. It can't run directly north south, or the valley would start in the Spearhead Channel. I also think it's very unlikely that they traveled all the way north of the Falls and then came back south to it, so that limits it to running southwest to northeast.
Paarfi says the Ash Mountains consists of three peaks, and these three peaks appear to border the Greymist Valley.
From south to north, we have Hanging Mountain, Gyffer's Peak and Round Mountain. Paarfi tells us no one lives in the Valley itself, and yet all three mountains appear to be populated (the passage above mentions that Gyffer's Peak is populated by coffee bean growers on its lower slopes).
Hanging Mountain has a village named Trader's Rock on its western slopes:
And people live on Round Mountain's lower slopes:
As they're passing Gyffer's Peak, Paarfi says mountains loom over them "on both sides." He also seems to believe that these three mountains are "together" the Ash Mountains, which suggests there are no other mountains nearby.
There's a little more information when Vlad and Morrolan make a similar journey:
I think Vlad's observation that the only way to get to the base of the Falls is over the cliff is interesting. It could be magic, of course, or perhaps something more mundane, like the Falls are formed when the Blood River flows into a hole/cave (something like
the Fell Brook flowing into the Gaping Gill, only much larger).
I also think it's interesting that while looking over the falls, Vlad says he can see "the occasional jutting of rocky peaks." This suggests there is more to the Ash Mountains than just the three Paarfi mentioned.
Trader's Rock's location on the western slopes of Hanging Mountain puts it on the western side of the Valley.
And Paarfi says,
Since the Blood River is on their right, they are on the western side of it, and have transitioned from Gyffer's Peak to Round Mountain, putting both of those mountains on the western side of the Valley as well. That means all three are on the western side, and something else makes up the eastern side of the Valley. My guess is that it's a relatively even ridge without much in the way of peaks.
I drew Hanging Mountain's southern slopes extending to a point 20 miles north of the Laughing River, and then guessed that Mount Klassor was about 30 miles from that point into the area on Steve's Map marked as the Eastern Mountains: