1) I feel that there are many similarities of the stages in Gilgamesh; in fact I wonder if this story has some origins of many tales of numerous cultural epics. Gilgamesh first encounters his adventure through dream, and Enkidu joins forces with him later, which introduces the great adventure to come.
2) I believe all four functions of mythology could be argued as occurring within the epic of Gilgamesh. It is undoubted that portions of this epic can be found in many ancient mythological themes and even in modern film or literature.
3) It is hard to say, but it was clear by the end that Gilgamesh had no need for eternal youth because he was now a man, full of wisdom and would not treasure such childish themes as eternal youth. Through coping with mortality he could better understand himself, others, nature, and existences worth.
I agree with you that Gilgamesh did not need eternal youth. I think that him gaining maturity was much more valuable than getting The Plant of Eternal Youth.