Is it permissible for congregants to pray behind the Imam at different elevations, such as some praying on the floor and others on a bed, if the space does not accommodate congregational prayer uniformly?

It is permissible for you to pray in congregation in the aforementioned situation. There is no impediment if some of the congregants are on the ground and others on a bed, as this does not invalidate the prayer. The jurists disliked the Imam's or the congregant's position being higher than the other's, though some said there is no dislike when there is a need for it. Prayer on a bed is permissible.