Having done some homework, there is no doubt about it, the shift register method is the way to go for reducing the number of output pins on a microprocessor. And the fact that the number of shift registers strung together is linear, it has definite advantages in operation as well.
I have trouble with arriving at the data based on long and short duration pulses superimposed on the clock input through an RC filter. My research says that it works but I don't know how I could output the difference in pulse width from the Arduino that the published information says is in the range of microseconds. That's hard to figure without an oscilloscope. But given the alternatives, using 2 pins (for clock and data) seems quite reasonable.
So a bit of experimentation will determine how I end up driving the seven segment LED displays. The good news is that 2 pins can do all of the displays. But I may decide on 3 pins just to keep the display sets independent.
Then there are the toggle switches and push button(s) which may require a parallel in serial out shift register or a resistor ladder to an analog input pin. I'll figure it out.
