Phonetic status of *y1 in Proto-Slavic

I wonder if the phonetic status of the Proto-Slavic *y1<*{u:,
(prologated)*U} is found out or at least a majority opinion exists.

An idea of it's bi-phonemic status seemes attractive to me,
considering:

1. A combined orthogram - actually <Ui> - is used in Cyrillic. It's
authors didn't hesitate to invent a completely new sign
for 'phoneticaaly monolithic' sound and never used a combined
orthogram in such cases (<oy> for /u/ being the only exception - but
they just followed a Greek tradition here).

2. Sound combinations whith adjacent *U and *i:*I reflected as /y/,
eg, in Russian (podymu' 'I will pick up' < *podUImo,). Proto-Slavic
*y reflected in the same way.

3. Early Baltic loans from East Slavic (Lith. mui~las 'soap' < East
Slavic *mylo < Proto Slavic *my1dlo, Lith. smui~kas 'violin' < East
Slavic *smykU 'the same') render East Slavic (etimological) y1 as
[ui].

Phonetic status of Proto-Slavic *y2<*o:n(s) is another interesting
issue.

It would be interesting to know what other members of the list think
of it.

Sergei