I have just done a little experimenting and I think I have some idea of why you're having a problem.
I opened up Mac Excel 2011 and selected a cell. I pressed Control-U and I was able to edit it. I also pressed F2 and I was again able to edit it. So I'm starting to think that there is a configuration problem somewhere on your Mac.
On Control-U: I am wondering if this no longer works on your Mac because you remapped Command to Control. 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. Therefore, Command-U (underline text) is now Control-U, and whatever was Control-U before (edit active cell) is disabled since a shortcut can only mean one thing at a time. That's my theory on why Control-U stopped working for you: You told the Command-U feature (underline text) to replace the former Control-U feature (edit active cell).
One possible way to fix this is that because remapping Command to Control eliminates access to Control key shortcuts, you could also remap Control to Command so that you can regain access to shortcuts that need the Control key like Control-U. Basically you would be swapping the two keys. (I think that would work, anyway)
But ultimately my recommendation is that if you bought a Mac intending to commit to the Mac...then really commit to the Mac. Stop trying to use the Control key and learn the Command key so that in applications that make use of both Command and Control shortcuts, you don't lose an entire set of shortcuts when you remap.
On F2: If pressing F2 changes your screen brightness instead of editing the active cell, then there is a setting you can change. Open System Preferences, click the Keyboard icon, click the Keyboard tab in there, and select the setting "Use all F1, F2, etc. keys as standard function keys." When you do that, F2 will now function as F2 and not as screen brightness adjustment. (I like some F keys to be function keys and other F keys to be hardware keys, so I use a utility called FunctionFlip to set the behavior of each F key separately.)
The other way to do that is to not change the "Use all F1, F2...as function keys" setting but instead press Fn+F2, because adding the Fn key always makes a function key do the opposite of whatever the "Use all F1, F2..." setting is.
You can try this out now. Select a cell and instead of pressing F2, press Fn+F2. If that edits the cell, the behavior is because of the Keyboard system preference.
