I grow e. amethystinum and yes, it does flop some even in full sun and sandy loam, never fertilized, no extra watering. It does seed around, but that does not bother me. Does look best massed. Single plants not very effective, from a design point of view. I wonder if cutting the flowering stems back by half a few weeks before they would otherwise flower might give a shorter plant that doesn't flop over? Has anyone tried this?
Sea hollies don't transplant well due to their long tap root. Vegetative propagation is by root cuttings. Seed is much easier. Available from Thompson & Morgan.