Stoneblade Mountain is an optional map. You don't have to pass there to complete the game (but it has some interesting stuff lying around)
Farlorn's Hope's portal should be closed from Greyfell until you access the platform from another location. However, normally it won't be much use travelling to Farlorn's Hope, because the access from Greyfell is blocked from the other side. You'll have to get to Farlorn's hope from another platform to open the passage.
