That particular AU situation is one I think of frequently, as well. Except I never really warmed to Marcel, partly because I was on the Shosanna/Fredrick train before even seeing the film, due to a promo picture of them I found on imdb.

And a big YES to all of that. Besides his sisters, Shosanna is the only genuine person Fredrick has known (and desired) since he enlisted and so to be rejected ruined him.