MIMF

Just to reiterate what Chuck was saying about the plans so it sticks in your head well... You apparently are looking down through the top with how the bracing is portrayed. When you are actually working with the braces (layout, gluing, etc.) your view will be from the opposite direction. All of your actual work bracewise will be a mirror image of what's shown while you are doing it.

I understand that builders use 2-piece backs due to it being difficult to get a quality board that wide. But is there another reason not to use a 1-piece if I can get a quality board that is big enough? Just wondering.

One reason is to book match the back. If it has interesting figure, you can book match it and make it more interesting.

You can place a center strip and align it with an interesting end graft on the end of the guitar.

Smaller pieces will have less force to fight if the board has a tendency to warp.

Using a bookmatched 2-piece back is the best way to have symmetry of physical properties.
I don't know if that's significant or not, but I suspect some people believe it is important.