**PROBLEM SOLVED**
As mentioned in my original post, I had set the programming for the four DMR simplex frequencies to talkgroup 99, but I could not get the radios to talk to each other. I've been fooling with the settings off and on for a few days, checking and double-checking each of the necessary channel parameters, all of which were shown as correct. For whatever reason (frustration, probably), I started re-setting / re-entering each of those settings in the respective drop-downs and downloaded the file to the radio once again.
Suddenly, now - it's working fine. All four DMR simplex channels are FB. My only thought is that the programming software might still be a little buggy and not totally ready for prime time.
Thanks for all of your suggestions, and keep a close eye on that Tytera programming software!
John AC4JK