If you are there at check in and hand your keys to someone else, you don't need to worry about the form. So that is irrelevant. Of course you can hand your keys to someone else if you are there personally. But, if you have two different reservations, you will only receive ENC for the room you are staying in. If you check in and then hand your keys to someone and you leave, well, you had to give the front desk your credit card (since they won't take someone else's if it is you checking in) and you are not entitled to any ENC for staying, because you aren't.
Regarding someone not named on the reservation checking in, what had been happening is that folks made a reservation for their MVC ownership; then instead of notifying MVC that there was going to be a guest checking in, they went to the BonVoy app to change the name of the guest, but they did not delete their BonVoy number. Thus, the person checking in had his or her name down on the reservation because of the change, but the system still applied the owner's BonVoy number and that owner was receiving ENCs from the stay even though the ts owner was not staying at his or her ownership. If the guest wasn't a BonVoy member, then they did not care about correcting the erroneous info. That has been a problem for MVW and they are actually auditing accounts, at least that is what was communicated to me.
The form isn't intended to prohibit last minute name changes as far as I'm aware; indeed, I have not seen any reports by folks who have changed the name in fewer than 30 days prior to check in by using the form where their guest had problems at check in. But I have read reports (I do not have personal knowledge) of instances where the guest was rejected because the form was not submitted, and the attempt was made only via the BonVoy back door access to change the name. I do not know how those instances were resolved; and as I indicated, I do not have personal knowledge of that happening, I simply read some posts in various FB groups about that happening, so they could or could not be accurate.