I also heard another issue with the potential or lack of for the 380 as a freight/combi was the main deck floor strength...do you have an informed opinion on that?
Not really - I saw parts of an analysis several years ago that explained why the A380 wouldn't make a good freighter. A large part was the MZFW was relatively low by freighter standards, so the relationship of payload to aircraft weight wasn't good - it could fly a long way but not carry lots of cargo. While there are exceptions, the worldwide cargo network is based on a max range of ~4000 miles (hence while passenger 747s routinely cross the Pacific non-stop, most cargo 747s refuel in Alaska).
Strengthening the main deck floor is pretty common when doing a freighter conversion but perhaps the large diameter of the A380 makes that a bigger problem.