You can prune boxwood but the rule is a little snip here and a little snip there, mostly applied to diseased leaves or to conform to a shape you desire. If you cut back to the bare branch you may not get new foliage sprouting out of bare wood and you will not like the new appearance of your plant. I wasn't entirely clear with my previous question. When I plant Blue Rug Juniper as a ground cover, I am concerned that weeding in between the juniper will be extremely difficult. No, I would not use black plastic.
Thick mulch ought to do the job better.
I have a question about Japanese roof iris. A while back I had asked if you thought this would be a good plant for growing under my Japanese maple and you responded that it should be okay. Japanese roof irises are seldom available in nurseries. Not enough demand for them, they say. I'd order them from a catalog specializing in irises.