Multiple questions, actually.
I have begun to connect the notes of a melody to the main chords that operate in each key, as a prelude both to accompanying the songs, and maybe doing a little improvisation. It has taken this idea a long time to get through my thick head. Questions:

How do you know when a song is heading off into a different key? Or, more subtly, presuming that the pattern of the notes is what is creating the different key as it evolves, how many notes do you need before you shift into the chords of the new key (supposing the notes are in a key not too far away from your starter key) as an accompaniest? Obviously you can leap in and state a new key right off, but is there normally a transition that alerts the ear that we are moving off into new terrain?

Next, what is the relationship between the standard patterns of chords (III-VI-II-V-I) in jazz, etc., and the movement of notes in a song? Do songs with those chords attached naturally have a melody that falls into that pattern? The books all talk as if this pattern is part of a powerful chordal system, but am I right in presuming that the real structure is in (or being evoked by) the melody?

yours, Peter T.