This is my take on the Mustang design.....
The two switches were chosen because it's much cheaper to start with. Those gibson style and blade style switches don't appear on a lot of electric and electronic goods of the period, if at all, but the switches on the Mustang, I remember mom having an old Box Fan from the 60's that had that kind of switch on it to switch between off, low, and high. They were actually rather popular for electric equipment in the 60's and 70's.
Some other models used that type of switch as well, Gibson used them on their non-reverse Firebirds for pickup selection, and I've seen them on some overseas examples from the 70's and 80's as well.
The reason for the placement was to make it intuitive to which pickup which switch affected. As these are student guitars, a lot of people new to guitar have no clue what those switches and knobs do. I know I did not till I actually started playing Electrics. Also, it did make the out of phase thing easily possible, albeit kind of redundant for most people as most people can't tell which pickup is out of phase and the subtle change in electricity on their ears. I can but it's so slight it's utterly useless to have that difference. Heck, I can't even tell which specific pickup is out of phase half the time when I have them out of phase, but I can hear slight differences in each individual pickup when I use them alone in different positions, to the point of not mattering as the difference is so subtle.
