I believe everything you just said is accurate, although we were not able to upload the old scanlist configuration file (the RSNetWorx version anyway) from the original gateway. We simply used the scanlist parameters we had some notes on, plugged those into RSNetWorx configuration you see above, and away we went!
Perhaps it was the fact that the web interface provides more configuration options?
Again, it is not clear why the attempts to configure the new gateway via the web interface failed - while the RSNetWorx configuration was successful - but the new gateway is now in service and we can reapply this solution to our other devices.
Thanks again!