But it's hard to see someone going into some kind of impossibly long and sad "OMG, my life is over, Rowan is the love of my life!" then marrying someone else <2 years later.

Not really. It's made pretty apparent in both the game and the books that marriage is more an act of duty than anything to do with romance.

The fact that he's in love with Rowan does not in any way negate the fact that he's suddenly got a responsibility to establish a dynasty and needs a wife to help manage the teynir he suddenly finds himself saddled with.

Love really has nothing to do with it. He needs to marry and produce heirs. It's his duty.