Is it necessary to re-format the card after the firmware is loaded in the camera? Or is it so insignificant that it doesn't matter?
No. Doing so does free up space used by the update file. Otherwise, having the file on the card does no harm, but, it is no longer of any use so why keep it?
Several Utubes on how to do. I’m not too great on the MacBook and you can’t do it on the iPad. I usually call canon support and they talk me thru the download process. You need a clean sd card. A fresh battery. You download the upgrade on your sd card( the hard part for me), then insert the sd card ( containing the download) in your camera, go to firmware update and follow instructions. When you start the download in your camera, don’t touch any button or handle the camera until down load is complete.