> > So it's either *-jo/je- with palatalisation everywhere or
> > *-o/e- without?
>
> Yes.
And we know that if we find root-final -s where it's expected,
it's because the 'suffix' is -o-/-e-, and if we find -s^ where
we would expect -s it's because the suffix was -yo-/-ye-, and
in no case is either the result of regularizing a paradigm
with alternating -s^ and -s ?