I hope someone would kindly answer the following questions.
In this song, there are the words "нижины (=низины)" and "ужины (=узины)" that are not found in ordinary dictionaries. Why is -жи- used instead of -зи-?
The last phrase is "Каравай, каравай, кого хочешь выбирай." I understand кого is a relative pronoun. If so, "который" seems much more appropriate than "кого". Why is "кого" used?
This song is used in the movie "Чебурашка", where children sing the first phrase as "А на Чебурашкины именины..." Is "Чебурашкины" an adjective of "Чебурашка"? If so, why is the word-end form not adjective-like?
A newly added question related to item 3:
I wonder what the difference between possessive and genitive cases is. For example, what is the difference between Чебурашкин друг and друг Чебурашки?