Update on my attempt to pay off the statement through XE. I am happy to advise that the funds have finally hit the account after 10 days. I still do not know what caused the delay, although XE seem to think it was because of the maiden transfer. They say future transfers should post within 2 business days.
The other interesting thing I noted was that the $20 fee wasn't taken by the intermediary bank, so the full amount was credited. Not sure if they simply waived this cos of the delay or if there is another reason.
I am happy, but more like relieved.