I love Opal, she's great. There's no way she's ending up with the Duke, based on:
1) Her dynamic with the childhood friend, they both obviously love each other but can't marry due to differences in status
2) Opal literally thinks this in a chapter ^ (she loves him but they cannot marry)
3) Her outright stating she is uninterested in the Duke. The way that scene is played there's no way she's suddenly going to fall for him lol