I had imagined that the Charr who initiated the searing knew full well that there was an extremely high probability of death during the event, but were being prodded and manipulated by the Titans to go through with it anyway. That and the Flame Legion uses their people as tools more so than any of the other factions. That and after so many years of humiliation and losses to the Ascalonians, I'm pretty sure that the ones who triggered the searing felt it was worth it. The bulk of the Charr population were up in the northlands and to the east of Ascalon at the time.
The second question regarding the landscape falls in line with so many other issues with GW2 that it's only the tip of the iceberg. The simplest answer is that with a z axis, Anet created a more suitable environment to jumping while still attempting to preserve features of the original game. Unfortunately there's going to be tons of discrepancies throughout. It's not a lore answer but the most accurate answer I feel you'd get.
The final answer is no, landscapes normally don't change that much in most environments. Yes weather and water erodes rock, changes rivers course and can wear down hills and mountains but not at the rate that GW2 would have you believe.
Man made intervention could alter most landscapes in the way you see,
Constant geological instability is a possibility but on a level that would make building large structures extremely unlikely.
Or you can say magic altered the landscape or just say the dragons did it...