What an episode! Perhaps this might help:
If the leak is now proven to be in the main drain line (as it appears, since shutting that line off from the pump makes the leaking stop as a whole), then it seems 99% the case that the leak in the main drain line is not as low as the actual main drain itself or even in the deeply-trenched pipe- It's in line with the skimmer opening, since that's where your water loss stops.
With that sorted out, it would only be question of accessing the main drain pipe that is near the surface, not too far underground... The material of your deck would determine just how much of a task that would be, but it's already much better than any main drain problem further below. It seems you're getting closer to the source, and the news so far isn't too bad.
PS. Poconos' post points to the possibility of a leak in a possible pipe connection between the main drain & your skimmer box. Did you check to see whether water continues to drop inside your skimmer box once the rest of the pool water stops dropping at the skimmer opening? His is a great question that might help you get to the right (& hopefully final!) fix. This is the 1% possibility that could indicate a deeper leak location, which might means looking at your options for either main drain & MD-pipe replacement OR just keeping the main drain shut-off.
Sorry I'm typing fast, so may not be as clear as I should be :-)
Best of luck.
Bookmarks