"do all designers think that what they do 1st time round is perfect and never to be changed?"
Aint that the truth. I am working with client right now that we have to keep coming back to the designer for more changes, blah, but hey that is life and the way both parties should understand it works.
As for
SEO leading the designer that is tough to say. You cant just give
SEO a blank canvas and then say "rank it". You have work through wireframes to get the look and feel the client wants. Work tightly with the deisnger to make sure the navigational arhcitecture is optimized and most importantly make sure the website owners have the "right" content in place for the pages to rank.
Who leads in this project is really a "it depends" answer and whatever works best for everyone.