the thickness of a single sheet of drywall
Did they happen to specify how thick that single sheet should be? I bet that they didn't and are being over protective, because they sell drywall in varying thicknesses from 1/8" up to 5/8".
But I don't think they're being TOO over protective, because I think that's the exact same mount I used, and it mounts using lag bolts. Lag bolts are very heavy duty bolts that are driven into the wood and bite into it to create a very strong anchor.
Their strength, however, is in the depth to which they sink in the underlying wood - the deeper they sink the more thread length is biting into the wood holding on tight. So - if you get too shallow of an installation, you don't get the strength you might need.
I read another answer where someone claimed they had a 50" TV mount just in drywall, no studs!
Not for long, I bet...
The other option I considered (although am loathe to attempt) is to cut away the extra drywall, attach some more wood onto the stud (enough to make up why width of the drywall I removed) and attach the mount to that.
You could do that - I thought about doing the same thing - and the way I would do that is to use a multi-tool (below) to cut out the drywall in the same shape as the back plate of the mount, affix that to the studs very securely, mark the stud location on it and drill through the plate into the studs and mount the panel.
HOWEVER - I suspect there's an even simpler solution.
Get longer lags.
They come in a wide assortment of depths and diameters. Just take the lag bolts they supplied with the mount, go to Lowes/Home Depot - and find one that's the same diameter as what they supplied and about 1" longer than what they gave you, and use those instead.
You don't say what size or gauge the studs are, but regardless, any reasonable stud is more than adequate for the weights involved. I'm not fully clear on what your connections really are, but in general they sound adequate. Are you sure the bulge occurred during the installation and wasn't always there? Drywall walls are rarely perfectly flat when checked with a straight edge. If the bulge were caused by the installation, it is probably the screws pulling the adjacent stud out of plumb due to some misalignment with the other studs and/or the mounted devices.
Metal studs are rather elastic, it doesn't take much side load for them to deflect a few mm. Despite my uncertainty of what you actually did, I'm fairly sure you have no structural issues and do not need to worry about anything failing.
Best Answer
A swiveling or articulating TV mount will tend to want to pull itself out of the wall when swiveled or extended, more than a flush-mount bracket would. Therefore, this situation calls for a bit of over-building to ensure safety, and to put up with the additional stresses of moving the bracket around.
You will need the following:
I suggest you pre-paint the plywood to match the wall or the bracket. Either will minimize the appearance of the plywood.
Center the bracket on the plywood and mark the holes. Drill holes big enough for the t-nuts and install them on the back side. Hang the plywood on the wall and mount it with the lag bolts or the deck screws. If you are using lag bolts, make sure to pre-drill the studs before installing the lag bolts. Since I can install a screw with one hand, I often use 2 deck screws to initially hang the plywood on the wall, then drill and install the lag bolts.
If you use only deck screws, try to find ones marked with a shear force rating and over build by a factor of 2-4. Or just install at least 6 or more per stud to be safe.