Speaking of "control"...one thing you might find is that an octagonal box or a standard square or rectangular softbox has more control than a shoot-through umbrella. One of the things that shoot-through umbrellas can tend to do, especially in smaller shooting environments, is to send 60% of the light forward and through the umbrella, and then bounce the oher 40% or so "backward"...this can lead to a staggering degree of what is called ambient spill light, especially in lower-ceilinged rooms, small rooms, and so on.
The octa, on the other hand, has a black outside; that keeps most of the light inside of the modifier. Looking at the setup as shown in the last image, the background flash would send some light toward the seamless, but would also likely shoot some light through the outer sides of that umbrella, as well as send some backwards, away from the backdrop paper, and toward other surfaces.
One "trick" to using a shoot through in a smaller room like this is to use it to create a main light, and to harness the ambient spill light as a source of fill-in light, for the shadows!
I hope you enjoy the new 59 inch octagonal modifier; that's a large enough light source that you'll be able to use in indoors in a lot of ways.
