I understand your confusion about omnium nostrum interitu. To be honest, I haven’t really memorized all of the uncommon forms (like the alternative genitives of the personal pronouns). I need to.
Why cant it be it in fact, and not only?
I’m sorry; I don’t understand the question.
The New Latin Grammar, in its § 141, goes: " These special forms are, in general, survivals of a very ancient form of declesion differing from that of nouns." So, although they dont follow any of the five declensions, they have one of their own. I guess, but havent checked it, it happens the same with the numeral unus, the adjective totus, and others that cant be fitted in any of the 5 decl.
Well, unus, totus, and those like it (UNUS NAUTA = unus, nullus, ullus, solus, neuter, alius, uter, totus, alterus) do follow a slightly different scheme, which is remarkably similar to many of the demonstrative pronouns (iste, ille, is, idem, etc.) But the primary differences are in some of the neuter singular forms (id, aliud, istud) and in the genitive and dative singular (unius, uni; totius, toti). Though I still forget the alternate dative endings (i.e. illi, NOT illo) on occasion, the fact that many of these are common forms makes memorizing them easier.
This isn’t quite the same as nostrum, though.
Best luck,
David