Some letters aren't letters

[Torsten]

Torsten: sorry to have to point out the obvious, but you can't argue it in both directions at the same time - either you think that y is a variant of a and o, or you think that o and a are variants of y. You say both...

"it is possible to replace each instance of [y] with [o] or [a]."

...and...

"word final [y] is a replacement for word final [a] and [o]"

So, which direction are you actually arguing for?

I mean that the scribe of the manuscript has used [y] as a variant of [a] and [o]. If this is the case it would be possible for us to remove all instances of [y] by replacing them with [o] at the beginning of a word and with [a] or [o] at the end of a word.