OK, finally full success to report, and the failures were all linked to
one very subtle and maddening typo: near the very end of the parameters where we specify the path to the photo attachment, I had accidentally entered a space in "
BlueIris" (actually it is there from what I copy/pasted from your first post, I only changed the Drive Letter without paying any attention to rest of the path).
So in my case, "C:\Blue Iris\Alerts\&ALERT_PATH", rather than the correct "C:\BlueIris\Alerts\&ALERT_PATH".
Once that was solved, I just needed to adjust the UI3 link portion of the parameter statement from "&WAN" to "&LAN" as I use a permanent OpenVPN connection on my mobile phone back to my home router. For my purposes, my B.I. server has a static private LAN address, regardless of whether I am inside or outside the home LAN.
I thank you once again for all your help, hopefully this troubleshooting work will inspire more people to pursue this very elegant route of getting notifications. One of the reasons I really wanted to get this working was due to the fact that I still can't get the official android B.I app notifications to "punch through" when the app isn't running in the foreground. If you have any patience left over to offer any further troubleshooting suggestions on that one, I'd love to hear your ideas. This is on a Samsung S10E, and I have verified that the app has been granted all the possible notification permissions, as well as turning off the battery optimization setting. The phone shows up in the B.I general "Mobile Devices" setting screen, and the checkbox is enabled for push notifications. I haven't setup geofencing, so it's not a problem with that. On the camera Alert settings tab, it is also Enabled, as shown below (basically the stock default settings):