I had quit QH long back and started after the new season started.
For some reason I don't think Aahil is Billi's anything because in all Gul's stories, even when the boy and the girl are fighting, boy's family love the girl and think she is perfect for their son. Billi would get a heart attack if she sees Sanam. But who knows, maybe GUL is trying a new angle.
Everyone might have thought that all died in fire accident and that is why nobody to help. You never know after 6 - 8 months into the show, they might bring new faces as grown up Haider, Humeira, Nikhat, Ayan etc. One never knows with these soaps. Anything and everything can happen. Smart way is not to use brain at all. We can at least enjoy it then. Hahaha, easily said than done.