Oh about the flipping back at will, I am not sure what you mean. I don't think it adds extra constraints too much so that's why I don't think we are thinking of the same thing. Think of it like you would really play the game in real life physically. If you flip over two cards, you choose when to flip them back over manually. The idea being once you flip any card over, you can't flip it back without picking a pair card. Once you've selected a pair, you just click anywhere to flip both back over to continue picking your next set, in this way the experience is smoother instead of having to wait for a fixed amount of time to be able to flip the next set. It also kind of emulates what it is like physically playing a match 2 game. You could even keep the timer that flips them back over automatically like you have but then they can still click to set that timer to 0 if they want given they've already memorized the given cards.
Totally get the time constraints, great job all around.