Vagelis.

Q1 You can't use the Node of the indicator directly on the screw you need something flat between the screw and the indicator node.

Q2 For measureing then you want over the full length of the screw. For straightening then it depends on the bend amount and where on screw it's bent. Often just moving blocks slightly in ward of ends and light pressure is enough to straighten.

Q3 Trial and error I'm afraid.? . . . How longs a piece of string.? Often just a little more than the amount of bend will do it but could require more depending on where the bend is on the screw.?

If you go too far then just tweak it back but often you'll creep up on straight. It's honestly not difficult to straighten screws just time patience needed. Don't be afraid you would have to be Very rough to destroy the screw. Just protect the threads while applying bending force and you won't have any problems and you'll be surprised how little it takes to straighten them.( Or bend them.!!)