I dug a little deeper, but have class in a couple of minutes, so won't be able to go farther.
Our financial aid script is pulling up the proper dates - even for concluded students. It uses a combination of student activity (analytics) and listing multiple submissions for a student.
Currently, the list multiple submissions (or even list a single submission) is turning up like the completed student never took it, but I'm getting a date from somewhere. I'm definitely starting to feel your frustration.
Quiz results for concluded students should not completely disappear once they're completed. @kona can confirm or correct, but I think we've gone into test or beta, re-added them there, and then obtained the results. By going into test or beta, they don't get the messages about being re-added to the course.