> With a single random password most people will write down their password, so anyone who can read what was written down gains full access. With a random username and a user-chosen password most people will write down their username but not their password. Clearly this approach is more secure.

I don't believe this is grounded in evidence. You are basically saying that given two hard to remember strings, most people will write down one hard to remember string and not the other hard to remember string. Why?

> I don't see how relying on haveibeenpwnd can be considered secure. Many people use the same password for different sites. If your site's login credentials are just email+password you are relying on the security and honesty of all other sites that use the email+password combination.

I think you are missing the point of the haveibeenpwnd service. The point is to block people from using ANY password that is listed in the haveibeenpwnd database, thus denying attackers from using that dictionary of known passwords.