3) What is the reasoning for doing hyphenation only after threshold=1
fails. Naive common sense tells me that if the user specify hyphenation
we should do hyphenation before finding line breaks.
Finding hyphenation points is time-expansive (all words must be hyphenated, not only the ones "near a line's end"), the sequence of elements becomes longer, there are more feasible breaking points, and a line ending with a "-" is less beautiful; so I thought that if a set of breaking points could be find without hyphenation.
I've just started to read Knuth's chapter on breaking paragraphs into lines, and from what I've read, he considers excessive hyphenation a bad form. The main benefits he gives for taking the entire paragraph into account when deciding where to break lines (as opposed to the more traditional just-look-at-the-current-line analysis) are a reduced need for hyphenation and a reduced number of over-spaced lines (i.e., too few words on a line requiring large spaces between them for the line to be justified.)