Will Fly - Old Time sessions can be found in 'The Bedford' on the right hand side as you go in the entrance from the Esplanade which go on most of the day. In the main bar on the left hand side there's always a completely mixed session going on all day until very late. There are an awful lot of sessions or singarounds in 'The Anchor' middle bar upstairs, The New Tavern (you'll see Leadfingers there), The Royal York and Faulkner, The Swan (both inside and outside), the Rugby Club, The Cricket Club, the Woodlands, The Radway and The Volunteer - just about everywhere you can think of.
Jack Campin - See Leadfingers link above. I personally do exactly what you are thinking of doing, which means I can see the shows I want to see, and still have a wonderful time on the fringe, but I always have believed in putting money in the collecting tins every day to support the festival, and I like the official campsite too. More playing there if you need it.