I don't know of a way around selecting text boxes without a mouse (and that's surprising because I'm a keyboard mover)
So, you may need to try to remove other elements first (carefully avoiding going over a text box).
Make a copy of the file first!
Then try removing "heavy" things like pictures and links.
Save
Close and Re-open
Now try selecting a text box.
If that doesn't work try this:
Right Click on the sheet tab (at the bottom) and Select "Move or Copy"
Mark the Checkbox "Create a copy" toward the bottom of the dialog box that appears
Then in the drop down menu at the top of that dialog box select "(new book)"
Save that new book, close the original file, and now see if you can work in the new file.