This Wiki article seems to suggest that words like makes had lost their final syllable schwa in normal speech already by Chaucer's time (palmeres > palmers is the example they give). The rule, as formulated, would also apply to the form maketh: here also the schwa is only surrounded by single consonants on each side. Does this mean it was [makθ] rather than [makəθ] by Late Middle English?
Another reason to think that this schwa was lost would be the evidence of English dialects that preserved the -(e)th ending: cf. wearth (not weareth) in the third line of the Yola "Maiden of Rosslare" poem here.
Is it reasonable, then, to say that this schwa was no longer pronounced by King James's time?
Maketh is almost universally pronounced [meɪkəθ] and not *[meɪkθ] in litugrical KJV reading today, so if I'm right, this sound had to reappear at some point. XVII-century evidence like Hodges' Special Help to Orthography also suggests also that the -eth spelling was originally ignored, with people freely pronouncing -eth as [z] or [s]. Would the introduction of the spelling-pronuncation [əθ], then, be a suitable explanation for the schwa?