'Disable telnet' toggle should work in switching the Telnet based collection on and off. There is a bug when accessed in Internet Explorer that the setting does not seem to get saved, UI switches back toggle. But it does save, can be verified using Firefox/Chrome.
'SMTP settings': Agree that we need to simplify this to not have to fill all fields to enable 'Save' button and a bug that it does not allow multiple email recipients. We will address this.
You could choose "diagnostic" from the Persona dropdown after selecting the required 'Type' and 'Subtype' in device-based data collection form. Few device types may not have a 'diagnostic' persona for collection. Is that what you are referring to?