1. Logging the IP address is not something Esri does. I have no idea why. It's lame. I don't think the auth requests ever make it to the IIS server log either, I have logging set to verbose and they are not in there.
2. You only need the portal admin account for a few administrative activities, like initial installation and maybe upgrades. You should assign admin privileges to a normal account (like, your own!). Once you have done that you can revoke admin privileges from the admin account until the next time you need it. You can also disable login for the admin account. They probably baked the actual account name into the configuration someplace so changing now is probably a bad idea. (I think you'd have to delete the account, I don't see a way to change the name. And it might happily delete the account when asked without telling you you would break things.)
Disabling logins on the admin account means no one will ever be able to guess the password. You just have to remember to re-enable it before you get 8 hours into an upgrade and find out it's unavailable.
