1. If the 50,000 points are shown as "pending" contact American Express customer service and ask if they can post the points now to allow you to transfer to Delta.
2. If not, just keep the Platinum Card open until the 50,000 points post, this will automatically repay the Advance, assuming both cards are linked to the same Membership Rewards account.
What happens if one closes the account while the advanced points haven't been paid off? Will AMEX charge the account at 2.5c/p and ask you to pay it off before the account can be closed?