Nope and nope. You can change cards when you return the rental car, but if you pickup with one/return with another, neither will offer coverage as they all say (that I've seen) that the rental must be initiated and closed with the same card. And you can either opt in or out of the insurance for the whole rental, not a partial. Seems like your best bet is to:
1. See if they will expedite your card to arrive sooner, or
2. Take the first car for a day (or until you have your new card). Then go return it, re-rent with the new card.
Good luck and sorry for your misfortunes!