Latin /a/ after labials, IE *mori

From: Andrew Jarrette
Message: 46941
Date: 2007-01-13

Awhile back we had a bit of a discussion about the idea of Latin <a>
after labials representing IE *o, the classic example being Latin <mare>
representing IE *mori (we also talked a bit about Latin /a/ sometimes
possibly representing IE *e).  But if this was a regular change, I have
found words which seems to be an exception: Latin <moni:le>
"necklace", from IE *mon- "neck" which is also found in English <mane> and Old English <mene> "necklace"; <moneo:> "remind, admonish"; <mons> "mountain", <mox> "soon, presently".  If IE *o regularly became Latin /a/ after
labials, shouldn't the Latin words be *<mani:le>, *<maneo:>, *<mans>, *<max>?  Or could the IE word for
"sea" have been *mari after all?  I'd like to point out that the Old
Irish word for sea, <muir>, has the form <mara> in the genitive singular and nominative plural, and the Cornish word <mo:r> has the compositional form <mari-> (according to Stuart Mann's IE dictionary).
Did Latin really change any *o after a labial to /a/, or was an IE *a preserved in *mari and other words?