RE 1. No, LOGIN method (be definition/spec/RFC) doesn't use base64.
RE 2. UseBestLogin uses a different authentication method (LOGIN, PLAIN, CRAM-MD5, NTLM, GssApi/Negotiate/Kerberos) depending on the server CAPABILITY response, it may also secure the connection with TLS/SSL in some situations.
RE 3. Yes, exactly. Here's how IMAP LOGIN command looks: a001 LOGIN SMITH SESAME. No Bases64 is used.