Can you replace the rear fingerprint sensor/power button?

As you have stated and reinforced by the links you provided, it sounds as if a production run of these devices were not given a quality control final check. This does happen often on phones, and you can replace them but only within the 14 day period (if you got it from a carrier), but if you are beyond that time frame then you will need to contact LG about it.

However, it is possible to replace nearly everything on the G5, and this does include the buttons. Take a look at the photo provided in the iFixit teardown: https://d3nevzfk7ii3be.cloudfront.net/ig...

On the left, you can clearly see that they have removed the fingerprint scanner. This replacement is possible but I would not go through all of that work just due to a rattle. If the rattle became worse and the button started to slip or give inaccurate readings, then I would contact LG for a warranty claim. If you do the repair yourself or have a repair shop do the repair for you, it is possible to tighten it up if it is still a bit loose. Each person would have their own opinion on how to complete this task but it is possible. Yet again, I stress that if it is just a mild annoyance and does not impact the use of your phone, just deal with it--or contact the carrier or LG to get a new device--but I would not attempt the repair to fix something that is not broken.