I don't want to distract you from the arguments you're enjoying on the posts beneath this one, but I have another example of Ramban disregarding and contradicting midrash aggadah to share with you.
Its found in his commentary on Chukat, (20:1 or in some versions 20:8) where he disagrees with Rashi saying
"These are divrei aggadah, but they don't clarify the matter"Ramban then proceeds to present his own reading of the verses, a reading that does not coincide with the Midrash's.
Tangential discussion here.
Search for more information about ### at4torah.com