After getting this far, it had to be some issue with mail being invoked by launchd, so I searched on that and I found this which referenced this. It was the .plist file in that it did not include the AbandonProcessGroup key which would allow the mail process to live long enough to work. Adding
<key>AbandonProcessGroup</key><true/>made it work.
I was setting myself up for this in that I was using a .plist generated by a 3rd party app to "save time". Such apps are not always complete or accurate in the .plist files they produce, apparently.
Thanks to @user3439894 & @Mark for their help.