I recently read Neil Gaiman's Norse Mythology. Although he's probably not considered the top authority on Norse Mythology, he did study the important works of Sturluson.
Here's a quote right form the beginning, to add to what you already said about Muspelheim and Niflheim (and this quote is on the back cover):
Before the beginning there was nothing - no earth, no heavens, no starts, no sky: only the mist world, formless and shapeless, and the fire world, always burning. - Neil Gaiman's Norse Mythology
And then you mention the creation of Ymir, and then his death by Odin and his brothers, who then used his body to create midgard... and so on.
I had to look at the book a second time to see if anything was mentioned about the creation of Yggdrasil, and nothing of the sorts is mentioned, perhaps only implied.
... is a mighty ash tree, the most perfect and beautiful of all trees: also the largest. It grows between the nine worlds and joins them, each to each. It is the biggest of all the trees there are, and the finest. The tops of it's branches are above the sky. - Neil Gaiman's Norse Mythology
What I gather from this, if it grows between the worlds, all it really is is connecting the worlds together with Midgard in the (well...) the center of it all. This is a geocentric cosmology. But there is more about Yggdrasil mentioned in the telling of Ragnarok. When Loki and Heimdall are fighting on the battlefield.
"The world is burning," says Loki. "The mortals are dead. Midgard is destroyed. I have won." "I can see further than you can, Loki. I can see all the way to the world-tree," Heimdall will tell him with his last breath. "Surtr's fire cannot touch the world-tree, and two people have hidden themselves safely in the trunk of Yggdrasil..." -Neil Gaiman's Norse Mythology
"I think the ash tree has always existed and it will continue even after ragnarok, because it simbolyzes the never ending cycle of life, that the world will always exists even after we are long gone"
Ragnarak has been viewed as the end of a cycle and the beginning of a new one, that there is no ultimate end. What I find interesting about Yggdrasil, is how it's mentioned as growing in between the worlds, or how it's branches are above the sky. It seems that whatever it really is, is what connects the worlds together (or dimensions).. and that the tree's seemingly indestructible and perfect qualities make it something that resembles some kind of etherial substance that links everything together... and is something beyond or limited to only one dimension. Perhaps the omission of it's creation is due to the fact that it actually isn't a tree at all... but the tree is the closest tangible "thing" to be used to symbolize it's characteristics and function.
