A snappy comeback

A long time ago I vented an idea here that perhaps the vowels of the
Greek alphabet were not reinterpretations of the signs for Phoenician
laryngeals, as usually assumed, but that Greek at the time still had
the PIE laryngeals, and that the signs for laryngeals were only
reinterpreted as vowels later, as Greek lost the laryngeals.

Jens recently, and (I think) Miguel then, sensibly pointed out that
the Greek letters for vowels were also used for the e/o reflexes of
the ablaut vowel.

But then I got to think of the fact that Greek has two vowels for
each of /e/ and /o/. Yes, they are used to distinguish length, but
were they always? The letters themselves (epsilon/eta, omikron/omega)
don't seem to be related.

One could imagine a situation where one of each pair had been used to
designate a laryngeal, the other one to designate a vowel (/e/
or /o/). Or?

Torsten