Yes, if you enter Hana GF path that means you are off Mina's path. Either voluntarily by telling her about it, or she'll dump your sorry ass next update, after having find out you've tried to make her a side piece.I'm not sure about this one.... As far as I know, you are then not on Mina's path anymore.
Telling Mina about Hana early is important too, but you still can't have Hana as GF and fool with Mina in any capacity, you'll have to break off the deal with her or face the consequences.I think this is it... Will give it a shot