I think I can guarantee that H. niger will not like being buried under 3-4 inches of even the best of soils. Still, I hate to recommend lifting it, as niger is a rather touchy plant compared to X hybridus. Sounds like you're between the proverbial Devil and the deep blue sea.

George

I would be more concerned with disturbing the surface roots of the Colorado Spruce, by removing soil. You will run less risk of impacting the tree by just top dressing your bed, if you really feel the need to. Helleborus should be planted and left alone. But sometimes they need to be moved, just remember large soil ball. The crown of the plant should be an inch below the soil surface, with an inch of mulch. Feed it with 10-10-10 water soluable fertilizer. Hope this helps.
Frank