If the assembly is made correctly, the Screen's front edge should be flush with the Columns' forward edges at each side. The Screen Frame should be wide enough that there is just 1/8" to no more than 1/4" spacing between the Frame and the sides of the Columns. If everything is painted Black (Wood Cleats) it should be impossible for anyone to see anything but what appears to be the Screen suspended in Mid-Air between the Columns.
Unless they get up to within a Nose's distance and then yeah....they might say, "I can see something back there..."
If they do,tell them to leave and never return, and go back to enjoying the fruits of your labor.
Now for my own take on the Right & Left Mains.....
I would place them outside the Screen so that you can have as wide a Front Sound Stage as possible. Visual Cosmetics aside, collapsing them in toward the center will reduce the scope of your aural imaging and lessen the impact when sounds travel from right to left across the Center. You don't have a lot of space.....but that shouldn't keep you from thinking your sounscape isn't spacious, so I'd place the Sub directly centered under the Screen, and spread the Right & Left speakers to the outside...with the Right set as deep into it's shallower corner and angled slightly inward, and the Left in the same manner and distance from Screen Center. Do that and the sound effects will be much more epic than it can or would be if the speakers are 3'-4' closer to each other and must be directed straight forward out of necessity.
In truth, it's only the Center Channel you should be concerned with...being able to center it side to side and top to bottom within the screen's area. Or as close to that as you possibly can. But of course all the above flies in the face of current popularity trends, where hiding everything behind the Screen seems the absolute status quo.
Well those of us who are old enough to know better.....well, we know better.
Unless you simply just don't have the space needed to optimize speaker placement, consigning speakers to a "homogenized" existence solely to be able to cram them all behind a 120" wide area is almost criminal. Yet it is done...all the time. And recently, all because of Acoustic Transparent Screens.
Ces't la vie.
At least some do build out entire AT Walls....but they usually do have more than 120" width to work with.
Digest a bit, and I'll come back later with a suggestion on Wall covers and fill-in behind the Screen.