How much of a difference does the range make with Mist-like skills?

Well, the range would certainly be inhibiting for a mage, but since monsters don't use magic attacks while in Darksight, and since Night Walkers can attack out of Darksight, I don't see the range being too much of a problem. You can simply walk across a whole platform dropping Slings and recasting Darksight while barely breaking pace or getting hit. I'm guessing that Slinging a map is going to focus more on coating the whole map with mist rather than placing it in strategic places, as F/P Mages do.

From what I've seen, monsters get poisoned in about 1 second, so unless the monster was especially fast, they'd get poisoned walking through a sling. I think that himes would be the only monster you'd train on that could run through it. And yes, they stick around even after you change channels.