I think I am struggling with something similar, but on a much longer list. I have a list of people, I want to check box against them, and then press a button to add something to their column. That is proving rather hard.
However, in your case, as you only have a limited selection, could you use user specific columns (one for each day), forward that data on when the confirm button is pressed, and then clear that data from the columns later in the same process?
I haven’t gone down this road, but I am fairly sure it could work.