The truck reveal makes no sense

The ending scene with its endless car-related problems just keeps on giving.

I was just thinking about Biff giving Marty keys to his truck that he knew nothing about. Why did they let Biff have these keys? So he could wax the car.

Why would Biff need KEYS to a car just to wax it, is beyond me, but I guess the family trusts this man that tried to rape the mom of the family now.

So the sequence of events..

1) Biff waxes the car (probably only one coat)
2) Biff locks the car
3) Biff closes the garage
4) Biff gives the keys to Marty
5) Marty has to open the garage door to see the newly-waxed car-truck

I mean, WHAT is the point of closing the garage door after waxing the car, then handing the keys to Marty, when you know Marty was just going to go and open the garage (wouldn't CAR-age make more sense?) door anyway?

Just useless wear and tear on the garage door for.. reasons? WHAT reasons?!

In any case, it makes no sense. Why would Biff even wax the truck IN THE GARAGE anyway? Wouldn't it be easier and less cumbersome to do it out in the driveway, just like the other car?

Do they only have one tiny garage for two cars, by the way, or do they just let Marty use their garage, but let their expensive main car just rot in the sun and rain?

If Biff DID wax the car outside of the garage, then what the F is the point of putting it in the garage and closing the door anyway?

Nothing about this ending makes any sense, but especially car-related things, holy cow!