Most of your questions can be answered by reading a map.http://mapper.acme.com/?ll=45.90793,-68 ... ahdin%20ME http://www.baxterstateparkauthority.com/maps/hiking.php
Book time = 1/2 hour per mile, plus 1/2 hour per 1000 ft gained. It's about 3800ft fromRoaring Brook trailhead to the summit, but you've got a little up-and-down over Chimney, Pamola, South, and Hamlin peaks: about 400' for Hamlin, much less for the others.
12 miles is about right (actually I think it's a little less, but let's be conservative) for the round trip from Roaring Brook, so 8.5 hours will have you covered at book pace. Note you may go a bit slower than book over the Knife Edge.
Katahdin is one of the finest hikes in the East. Similar to the northern Presis in the amount of above-treeline area, but with a bit more of a wilderness feel. (Also, it's more northern - watch out for snow by Columbus day!)
This may all be moot depending on whether you get to the road early enough in the morning, and how many other people show up on Columbus Day weekend, and whether the rangers close half the trails due to snow. See the other Katahdin thread started a week ago.