Few comments:
(1) There are several OneWorld lounges in Changi but they do operate on different hours and may not be open when you are there.
(2) I just flew several domestic and international segments on MH as OneWorld Emerald (AA EXP). I had no problem with the website accepting my AA number, crediting miles in a few days, and also allowing for selecting seats in advance. Also, I was always able to check-in on line before the flights.
(3) We only traveled with carry-on bags and laptop cases so I cannot comment on check-in luggage.