> I don't see how you explain Slavic g. pl. from *-oom > *-o::m.
Slavic -7
> looks like plain PIE *-om to me.
Probably, but I just tried to play with the idea that supelong vowels
were automatically narrowed at some stage of Proto-Slavic, thus *-
o::m > *-u:m. *-u:m > *-um looks like a regular Slavic development
(do you have any objections?), as well as *-um > *-U.