Samuel,
Thank you for taking the time to reply to this, it’s appreciated.
For #1, I was having trouble getting emails to be sent to [phone-number]@mms.att.net which is basically email to MMS. The reason for that was the absence of Reply-to matching From. I resorted to writing the wp_mail() variables to file to see what the email source would look like, and that’s how I figured it out. But now I doubt it’s your plugin, it most likely has to do with the way wp_mail() is called in this template.
#2 & #3 are oddities. What you say makes sense, and that’s what I saw in your plugin’s code. So it must be some middle-man function that intercepts wp_mail() request before it gets to your plugin.
Have a great one!