Thanks for the replies.
After trouble shooting via every suggestion posted and then some, I finally figured out what the problem was at about 7 am.... **Stupid User....**
I added a backing audio track (captured from the house mixer with a C-Role camera) into the sequences. Initially, I lined up this track with the Multiclip, and then disabled the C-Roll video track. This allowed Multitrack playback just fine. However (for whatever reason), I decided to change disabling the video track to enabling the video track and reducing its opacity to zero, which disables the Multiclip Playback...
The "Ah-Hah" moment was when I was scrubbing through a portion of the sequence that did not have house mix, but did have a lower-third, and the viewer went from Multiclip Playback to Full Screen...
Anyway, so as not to "accidentally" cause this problem again, I deleted the video track of the house mix, and all is good in my world again.
BTW - I know that I could have created the original Multiclip with the house mix track, but I had already created the Multiclips and started editing when I received and subsequently captured the C-Roll tracks.
Thanks again for the replies. All were greatly appreciated!