I can confirm that you can use your $450 credit to book a flight for another person as I did this myself last week. Note that you must use your Qantas AMEX card when using the credit (it doesn’t let you pay the remaining balance with another card).
I found the pricing on the AMEX travel website to be the same as Qantas website or lower, in fact I actually found business class sale tickets on the AMEX website that were cheaper than the sale tickets on the Qantas website so I received around $600 off the sale price on the Qantas website after applying the credit!
If you log into the AMEX website you can see the expiry date of the flight credit.
All the best,