Mileage upgrades do come from the same bucket as the automatic upgrades so if you do get to the Gold window and didn't get the automatic upgrade, you probably wouldn't be able to use miles.

As stated above, as long as the miles are coming from your account, there is no copay charged. If you were on the same reservation, I believe you would have to split the reservation if you wanted to upgrade one of the passengers but not the other.

The Caribbean price of 15K miles would only cover the BDA-PHL leg. The mileage upgrades are done by segment so you would need another 10K each way to upgrade the PHL-LAS segment..

Just a guess but I'd say your chances would be pretty good to get the Gold upgrade on the BDA-PHL leg for you and your friend since that's primarily a leisure route. The PHL-LAS segment is probably going to be a tough one for even one passenger.