The Titles of the Psalms: Original?

I've gone back and forth while contemplating and studying the titles of the psalms (i.e. Ps. 18's For the choir director [NLT]).  Were these titles written by the original author of the psalm?   Are they original or later additions?  Are they canonical and inspired?  By way of summary, here are what a few psalm scholars say. …

To the Four Corners of the Earth

Psalm 107 is is a liturgical type psalm according to Westermann and others: there is clearly an introduction, four "stanzas" each with its own thanksgiving chorus, and the entire psalm is closed with all the people voicing praise together for Yahweh's wondrous works (pala) of deliverance.  This is his loyal love (hesed) on display. In …