Is it possible for a glyph to actually “revert” the cursor to a previous position… that is, to the left?
I know zero-width is possible, but what about “negative width”?
Yes. All combining diacritical marks have a negative left side-bearing, and zero advance width.
That is how we get ä from a and ". The font combines (composes) accented glyphs from a base glyph and a diacritical mark. Many fonts position the diacritical marks using anchors on the base glyph and on the diacritic.