According to Robert Bringhurst, author of The Elements of typographic style, considered by many as the typographer's bible:
Anything from 45 to 75 characters is widely-regarded as a satisfactory length of line for a single-column page set in a serifed text face in a text size. The 66-character line (counting both letters and spaces) is widely regarded as ideal.
There is web-applied edition as well, where I believe you should find the answers to your other (1, 2) questions.
EDIT
There is a similar question with some comprehensive answers.