The easiest way for non-flying non-teleporting PCs to get to Lake Stormunder from Sandpoint is to take a ship to Brinewall and then just go up the river. Sadly, Brinewall was found completely abandoned twenty years ago.
Is it still abandoned? Surely some brave looters and squatters have moved in. Any thoughts on how to run the arrival? Are sea captains even going to be willing to dock there? (I'm guessing that with enough gold they'll be able to find someone who will at least get close enough that they can make it via longboat.)