If the silluq follows a vowel and there is already a note for this syllable, then the silluq is a metheg regardless.
Note re tightening up the vague decision on what is a metheg.
Sorry folks - just discovered that each of the three books begins on a note other than the tonic (and that's not with gin). Of the 21, only Deuteronomy begins on a note other than the tonic.
The implication is that the three do not stand alone. They are commentary. They are keys.