That's a really good point, Nike, well, two actually.
Marlene keeps proclaiming her love for Tristan, but the connection is barely there, it's almost like she's going through the motions, acting like she's supposed to - sure she likes him, but I've never thought that they have any kind of sizzling chemistry, what they have is not nearly as strong or palpable as the chemistry and sexual attraction between Marlene and Rebecca.
Which brings me to your second point - Marlene really is herself when she's with Rebecca, at least when she doesn't worry about the circumstances or the consequences of their attraction and love for each other. She's undiguised and leaves all that bluster and bravado behind which she usually uses as a diversionary tactic. She dares to reveal her true self, her insecurities, her deepest emotions - which is very touching actually. And like you say, Rebecca challenges her, she sees her real motives and dares her to be honest and face her fears, etc.
Rebecca is definitely the better partner and lover for Marlene. If only Marlene realized that.