As for the general question, you need to start travelling by the expiration date of the voucher. Otherwise, it will become void. This means you cannot use it for your christmas trip.

1. Yes, you may combine these two.
2. No, multiple customer care / DBC vouchers may not be combined.
3. If your transportation credit is tied to your SkyMiles number, or if you have an e-ticket number of the credit, then you can use it on Delta.com. The DBC credit cannot be used on Delta.com.