Hi Vivian,
Those are all good questions. This is not a simple project, but it can be made a little simpler by going at it in steps. Get a prototype that gives a number related to skin fold thickness. Then develop that further to improve the user interface and add more "bells and whistles."
There are several project management techniques, such as flow charts and fish diagrams (aka, critical pathway). I prepared a
hypothetical critical pathway chart for you. I am sure it is incomplete and has errors. It is just a rough example of how to approach something that has so many possibilities and potential paths as this project has.
View attachment DigitalFatCaliper.pdf
I suggest that you look at it, decide what really needs to get done first, and put some of the other things on hold.
Let's take the charlieplexing and pincers thing. Absolutely, you will need to know how that is wired at some point. But, do you need to know that before you get your feet wet in programming? Similarly, for programming, a very simple program would give you the pincer opening and/or fold thickness without all the data entry refinements you have mentioned.
I suggest that you get a feel for microcontroller programming first or develop a collaboration with someone who already knows how to do that. (If you use a chart like I show here, add that collaboration to it.)
As for the pincer and charlieplexing, I do not think an accurate schematic can be made with the information we have. Can you get local help? How far are you from Northeast USA, Australia, UK, or anywhere there might be a local member of ETO who could help determine the actual circuit for you?
To reiterate, if you are planning to do the programming yourself, I do not think the actual circuit for the pincer should be your focus right now.
John