Yes, they should match up nicely.
The mounting screws are 3 1/2" long
Yes. The plate fits the 3.5" boxes and 4" boxes.
I don't work for Home Depot, but when I ordered these, they were only available in white and they were recommended for some LED strip lights I purchased. As I mentioned in my review of these plates, they aren't large enough to completely cover some boxes, which kind of defeats the purpose of trying to camouflage the box. If it's large enough to cover the box you're using,, you might consider spray painting it with a coordinating color.
It's 7/8" like most NM cable clamps and knockouts on strip lights.