The way to get the link is:
Select your username (the avatar in the new not-technically black bar)
Click "My Logins"
Click "Add more logins..."
The answer to Can I (and How do I) use my askubuntu account on other Stack Exchange sites? is actually still correct, but Stack Exchange has changed their login page some so it isn't quite clear.
On the login page you need to click "more login options" and then input your launchpad URL in the OpenID field.
Yes, it should.
However, you cannot use your Launchpad OAuth login on any site other than Ask Ubuntu. So you should also add either a StackExchange local login or a secondary login from, say, Google or Facebook, in order to authenticate to other sites (when cross-site cookies don't work or such).