When playing co-op does everyone need a copy? And how do the saves work? Auto save at checkpoints or something else?

Some random people have said it some dates in the other Thread, but when we got our last date aswell as one other person confirming it. Then Valve comes and locks the thread, they are hiding something i know it.

The "HIDDEN" password puzzle makes no ♥♥♥♥♥♥♥ sense. Is there an error here? 1) The first picture on the board is the box from above. So the first wire stack in top-to-bottom order is (A E I M). In the picture from above, the upper wires (meaning column A E I M) are not changed. This is also the correct interpretation from the real solution. How the ♥♥♥♥ does D move to column 2 from column 4? I don't see how the logic mak...

The puzzle of moving the LED cubes into their colored spot ? Yeah don't be adding anything like that again in future please. Doing it once was enough, but 3 times to open all 3 latches on the safe to open it ? come on bro..... Took me at least an hour to open all 3. I calculated i was about 7 minutes away from needing a new monitor...

It' be awesome if puzzles could be reset so if you mess up, you're not stuck for half an hour or more trying to unfutz what you just did (or, if you're stuck and have been for ages, you can reset and then look at a walkthrough/solution).