The fields requested in creating a new newsletter should use inline help information to explain to users what is being requested for each field. Noting this inline would be helpful to publishers not aware of this limitation.
Yes, this would be a good improvement. Would you like to volunteer the texts, so a Google-Code-in-2017 student can make a straightforward implementation?
Maybe some strings of https://www.mediawiki.org/wiki/Help:Extension:Newsletter can be recycled / adapted?