Because in Mac Excel, Command-U (apply underline formatting) and Control-U (edit selected cell) are two different features, so if you remapped Command to Control at the system level, Excel now behaves such that all Command-modifier shortcuts are now Control. On Control-U: I am wondering if this no longer works on your Mac because you remapped Command to Control. So I'm starting to think that there is a configuration problem somewhere on your Mac. I also pressed F2 and I was again able to edit it. I pressed Control-U and I was able to edit it.
I opened up Mac Excel 2011 and selected a cell. I have just done a little experimenting and I think I have some idea of why you're having a problem.
