Datapoint for using pay with miles instead of eCredit:
28/12 - bought a $248.40 ticket in main cabin, using 5000 miles for $50 discount, and paying the remaining $198.40 with my Amex Plat
30/12 - charges posted on my Amex account as additional collection
31/12 - fee credit meter changed under the benefits page, and credit posted
Will do this again in a few weeks. Very happy with the free $400 DL eCredits!