I must live in a different reality from most people, I never really thought of the age gap as a huge issue. Someone very close to me is dating a person 15 years older. I personally am currently dating someone 6 years older than me, and to be honest I can't really picture myself dating people my age in the foreseeable future, her maturity, experience and openness is such a breath of fresh air. Sure, in the beginning some of your friends and family might give you and her some weird looks and comments, but in the end if those people really care about you, they'll just be happy that you're happy.
So with Bella I don't really see the age gap being such a huge problem, it's more of an opportunity for MC to grow up. The real issues will be the marriage, the teacher/student thing, and most important of all, if Bella and/or MC really love each other or not, because if one ends up with feelings that the other cannot reciprocate it'll just completely destroy their relationship.
As things stand at the moment I don't really see Bella's actions towards MC as him being just the "rebound guy", the way she's acting, specially from EP7 onward it really looks like she's falling for him, but I guess we'll only be sure if/when James returns and we find about whatever trauma she has.