Hi Cutt,
Your analysis is quite extensive, and I hope it somehow helps us get a version that works correctly. It would be great if the people at Microsoft used your information, since (as a developer, I know that) more information is always better, when troubleshooting.
From the information presented, you make a good case that the cause is a timing issue with the (Windows 10) "Your Phone" app -- that it's not getting a "message sent" acknowledgement before its (whatever) timeout, and so it's resending the message. I've
seen many posts from people who've replaced the built-in Samsung messaging app with the Google Messaging app, and their problem went away. (Here, "replaced" means that they installed the Google Messaging app and selected it as the default app.)
For me, that didn't work, and I think that the Samsung app is (at least partially) causing the problem. It's possible that the Google Messaging app is getting (and correctly forwarding) the message request from the Win10 app, but that the Samsung app is
still actively processing some messages (even though it shouldn't be). The only
real way for me to test this would be to (somehow) disable it or ideally remove it, but both would require that I root the phone. Sure, I could do that, but I'm not sure it's that important.
Not yet, anyway.
As always, all thoughts are appreciated.
Happy Tuesday!
Dave