> Is there an explanation why in them. pl. nom. PIE. -o-i gives PSl. -i
> instead of regular PSl. -e^ as in them. sg. loc.?
There is an interesting book by Robert Orr, "Comparative Slavic nominal
morphology: a new synthesis" (2000), which discusses just about
everything connected with the Slavic "Auslautgesetze" and allows you to
make up your own mind.