If you do not want to disable editing directly in cells, there's this long workaround:
If the cell is empty, it's easy: Press F2, then Ctrl+A.
Unfortunately, that shortcut only works if the cell is empty. If the cell is not empty, you must first clear the contents. So you could use this key sequence: Del, then F2, then Ctrl+A.
But assuming you want to retain the original formula, you'd have to copy and paste it.
Thus, if the cell is not empty, this key sequence does the job:
- F2 (modify the cell in place)
- Ctrl+A (select the formula)
- Ctrl+X (cut the formula to clipboard)
- Ctrl+A (jump to formula bar)
- Backspace (remove equal sign, since it is already on the clipboard)
- Ctrl+V (paste original content)
Note that steps 2-4 can be reduced to a single step: Ctrl+A,X,A.