The so called 'finger in the ear' is as others here have said, a method of staying in tune, especially when unaccompanied; it is a millenia old technique used by singers all over the world for exactly that purpose. There are 18th and 19th century woodcuts of London street singers and broadside sellers using it, also of Bengali temple singers and Eastern European peasants - some of the most skillful singers in the world.
Either MacColl or Lloyd introduced it to the revival - not sure which, they both used it.
The Watersons sang regularly with BOTH hands cupped over their ears.
Saw Dylan (sorry - Zimmerman) use it once but it didn't work for him - he could never sing in tune anyway.
MacColl's habit of singing with his chair back to front was a method of relaxation which allows a free, unrestricted flow of air, particularly useful for long-line songs. Have seen West of Ireland sean nós singers use similar techniques, usually side on with an elbow resting on the chair back - wonder if you think they were prats too - or is it just another case of corpse-kicking (he's been dead twenty years now - so you're perfectly safe; he won't come back and bite your bum)?
Jim Carroll