jholl, I now have. I was focusing too much on just the Checkout button.
As you were clearly stating: if I add cart attributes and/or a note to the cart, and then navigate away somewhere else, those attributes are indeed there when I go back into the cart, and are then captured/carried through and show in the completed order. Thank you.
However, if I add the various values in the cart for the first time, and then click directly and only on Checkout - without navigating anywhere else - the values are not saved/carried through. Different behavior when clicking on that button compared to leaving the cart via any other route.