On the subject of the young Kirk saying "Siberius," I would suggest that it is merely a matter of pronunciation since his first name ends with an "s" and the names are strung together closely. When I listened to it over again, I clearly heard a "T" in there.
Often, we hear things a certain way because our brains automatically look for certain patterns. For instance, when I hear the name of the toy company "Jakks Pacific," I can't help but hear "Jack Specific." It's just human nature. Besides, there are plenty of other undeniable references to Kirk's middle name being "Tiberius" (that is, as long as we overlook the middle initial of "R" on the tombstone created for Kirk by his old pal Gary Mitchell in "Where No Man Has Gone Before.")