I'm making some assumptions because we really don't know much about Drew, her affair with David, or her true goals. I don't think Drew would be interested in Kevin at all, when he's unattractive and not much of a "conquest". I guess I can see Drew playing a long con, but was Kevin's family wealthy? I don't remember that.
Kevin losing weight is pretty implausible. I suppose finding true love, and/or a near-death experience
might finally motivate him to do it, otherwise he's unlikely to change.
As for Kevin becoming trans, I just can't see it. His personality is pretty well defined, and he clearly lusts after Aliza and Ophelia. Kevin showed initial interest in Drew when she arrived, but once he learns Drew has a dick, I expect Kevin will give up on her.
I guess the overall idea just feels like it makes too many leaps (i.e. "what if?" & "what
if..." & "WHAT IF!"), to the point where it becomes a different story. Besides: this scenario wouldn't meet the submission requirements of being concise, and open-ended. What If submissions are supposed to serve as a starting point for a mini-story, not be entirely constructed narratives.