You guys did the right thing, Shut it down and check everything...
The condensation is from the warm gases escaping into the hopper due to lowered draft up the chimney, picking up any moisture from the coal and then cooling and condensing onto the lid or hopper. Normally a draft problem when this happens. The gases are not going up the chimney. The pipes should be cleaned every season after burning and draft checked periodically.
There is a Draft Manometer loaner program that you can get the device to check your draft to make sure it's good and setup correctly.
Do yo have your CO mounted near the stove, you might get another upstairs too.
I don;t think mine has a ash pan gasket either.
I remember we just talked about this the other dayAsh Buildup Problem: A Safety Issue
How did you make out?