Editing and sending
To change the editor, check out Customizing editor.
The quote strings can be customized in the configuration options: mail.reply.quote_line and mail.forward.quote_line. Both of these take two optional arguments %1, the sender, and %2, a formatted date.
The quote string is passed through Glib::DateTime::format() so you remove %2 and specify your own date format if you want. Note that the leader in the ..::format() documentation is the usual %, not \ as is specified there.
See Astroid setup for notes about configuring msmtp or sendmail.