Excel keyboard shortcut to deselect all

Paul's suggestion is good or you can write your own macros and assign shortcut keys

 Sub makeUPPER()
  ActiveCell = UCase(ActiveCell)
 End Sub

 Sub makeLOWER()
  ActiveCell = LCase(ActiveCell)
 End Sub

I provide you with two methods to accomplish what you want. Unfortunately, both methods do have their own caveats, since Excel does not provide an all-in-one keyboard shortcut for auto-fill. I highly suggest you try both methods and then determine which works best for you. Outside of these two methods, I do not believe you can accomplish what you seek without the use of macros or add-in modules. Should you go the macro route, the AutoFill method on any Range object should be all that you need.

Method 1 (alternate, but it works with no mouse interaction)

Another way to accomplish this is to use a combination of the fill-down shortcut and the select all in range. After you enter the data in the cell, press Ctrl+Shift+End to select from the current cell to the end of the range that would be auto-filled. Then, press Ctrl+D to "fill down" into the entire range from the top cell.

Method 2 (my preferred way, one-time mouse interaction)

Once you fill in the cell you want to auto-fill downwards, double-click the bottom right corner as usual. To repeat the "auto-fill down" step, so long as you only enter data and press enter, you can repeat it by pressing Ctrl+Y.

This means that you do need to use the mouse, but only the first time. All subsequent work can be done purely with the keyboard, so long as you only use it to enter data (i.e. you don't use the context menu button to modify anything).

Note that the first method does come with a minor caveat - your view/cursor will move to the bottom of the spreadsheet (which does not happen with the Ctrl+Y method). You can press the Home key to move the cursor back up quickly.