I had this happen to me twice and the solution for me was to delete the wifi network and keychain password.
The first time i fixed it by restoring from Time Machine which took a long time. I also tried the renew DHCP, delete com.alf.plist etc to no avail. The second time I found a better solution from someone on this forum:
Go to Network Settings
Turn Airport Off
Click Advanced tab
Select my wifi network and hit minus sign to delete
Go to Keychain access in Applications/Utilities
Find your network name in the list and delete that password login
Then go back to Network settings and turn airport back on
Click plus sign then Show Networks
Choose your network and re-enter your password
Then airport should work again, this is what fixed it for me.
All the best
I know you seem to have got things sorted, but for others for whom this may not have worked, here is how I got over the "self-assigned IP" problem
with airport selected, go to "advanced" and then select TCP/IP
In the "DHCP Client ID" box (which is usually empty) put any random string of numbers
THEN renew the DNCP Lease,
It worked for me after trying a lot of other ways
i am one of the many hundreds victims of this unbelivable nigthmare "self assigned ip" who finally got a solution with this trick.
Hope this work for others as well. By the way .. do you have an explanation for this lack of serious answer from Apple???