I received MBP as Christmas gift, still sealed; never opened the box. Is my only option trade-in at an $800 loss?

Either that or sell on your own. Return with receipt for a full refund can only be done 14 days after purchase. Being sealed in an unopened box may be helpful in getting a higher resale value when sold on your own, but will not make a difference for a trade-in. Apple will only be able to sell the Mac as refurbished and would not be able to sell it as new again.

Apple extended to return period, normally only 14 days from purchase. You had until Jan 20 to return for full-value refund. But that window has closed now.

You may find that Apple will NOT take that Mac for trade-in.

They generally do NOT take Macs currently for sale as new.