Oh man, I am sorry to hear about all of the trouble you have gone through! A few weeks ago, I told you that I would check out FT on my new iMac Pro and let you know how it goes. I'm sorry for just now getting back to you. I was on the road for the last two weeks and just got back this morning.
Unfortunately, I am in the same boat as you. When calling someone on FaceTime that is not connected to my home network, I can hear them, but I cannot see them (there is no incoming video feed). The other person can see and hear me just fine. On the other hand, if the person is connected to me home network, then everything works just as it should. These are the same symptoms that you had as well.
Yet another iMac Pro bug. This really bums me out as I often use FaceTime for business calls. I wonder if Apple is going to address this relatively soon, or will it be broke for months and months to come. I really am left wondering because there is an extensive list of iMac Pro bugs and we're 120+ days into the release of the hardware but have not seen much from Apple yet nor do we have any idea of what they are working on/trying to fix...