@strygo: I actually thought I posted the update…maybe it didn’t go through.
In any event, the replacement board from GGLabs did the trick. I believe something may have affected the original in transit.
Upon connecting the new board, 80 column mode worked right away from my C128 through the OSSC!
This device also worked for my Tandy 1000HX producing a clear, bright picture from the CGA output.
In regards to your second question, I would assume 75 ohm output too. Unfortunately, I can’t seem to get to the technical documents online at the moment. When I can, I’ll let you know.