I think this is somehow covered in the instructions for manual installation and you should still be able to use fastboot to manually install. However I would not go from 5 to 9, but step by step from 5 to 6 and so on…
- Your new display stays black
The procedure is the same in both situations. However, if your new display stays black you will not have visual feedback of what is happening in your Fairphone 2 during the process
For official FP help you could open a ticket in the bug tracker, the developer would know better if you can skip Versions or not.