Continuity/Lord Cambarth and Mount Lethe

Lord Cambarth is said to be from Cragstone. The virindi that interacted with Cambarth called itself Lord Sarvaen, of Arwic. Asjan's home was noted to be Qalaba'r.
Lord Cambarth used his own calendar system, rather than the commonly used Portal Year system. He measured time in years of exile. The first entry in his journal is dated 4th of Frostfell, 13th Year of Exile. The next entry is dated 7th Frostfell. No year is given, but we can assume this is also in 13th Year of Exile, just a few days later. Much of the middle of the book is missing, but in the latter half, the only entry is dated 22nd of Seedsow, 15th Year of Exile.
On the charred sign found in the Mount Lethe magma tubes, foreman Brelax, left a notice to all workers, dated 25th Seedsow, with no year given. Based on the context, we can assume this was three days after Cambarth's last journal entry.
Cragstone is the 2nd Aluvian settlement, after the Underground City, and it was founded in 0 PY.
We do not know when Arwic was founded, but evidence indicates Holtburg was the 3rd Aluvian settlement, and we know that to be founded in 2 PY. We also know that Harlune once lived in Arwic, and Celcynd became his apprentice, and during or after his apprenticeship, Celcynd first mastered planar magic. Celcynd used this planar magic in 6 PY to save the childen of Holtburg. So we know that Arwic had to be founded some time between 2 PY and 6 PY.
We do not know when Qalaba'r was founded either. However, it had to have been at 0 PY or later, as that is when Gharu'ndim first arrived. And we do know it was founded earlier than many other Gharu'ndim cities.
Lord Cambarth's calendar system most likely refers to the number of years since his arrival on Dereth. The earliest possible time he could have arrived would have been -15 PY, as that is when the first Aluvians arrived.
However, it is not possible for him to have arrived in -15 PY, given a few of the facts we have: He is from Cragstone, which was not founded until 0 PY. But if he arrived in -15 PY, the earlier entries in the 13th year of exile would have occurred in -2 PY. So that places his earliest possible arrival at -13 PY.
And if Sarvaen claimed to be from Arwic, then Arwic had to exist in Cambarth's 13th year. If we assume the earliest possible founding of Arwic in 2 PY, then Cambarth's arrival would be in -11 PY. And if we assume the latest possible founding of Arwic in 6 PY, then Cambarth's arrival would be in -7 PY.
And the latest he could have arrived would be 15 years prior to when he wrote his last journal entry. If we assume that it was written on the 22nd of Seedsow, 10 PY, that would put his arrival at -5 PY. 
Sources:
* Of Lord Cambarth
* Torn Journal (First Half)
* Torn Journal (Second Half)
* Mount Lethe Directions
* Mount Lethe Rumor
* Charred Sign
* AC:DM CD Lore/Early Dereth Texts/Of Lord Cambarth
* AC:DM CD Lore/Early Dereth Texts/Mount Lethe Rumor
* AC:DM CD Lore/Early Dereth Texts/Lord Cambarth's Journal