Did you find your problem? From reading your original post, you appear to have correct voltage. If you want to further check your coil you need to check using the ohms scale on your meter with all wires removed from both the + and - terminals. Appears you are trying to check continuity using your voltage scale. That won't work. The details you give appear your problem is not from losing a ground connection when switching the key switch on/off. Appears you are incorrectly checking voltage on which, in essence, is a single leg of a circuit (open circuit). The circuit to ground is only completed when the points close. From what you describe your coil is probably fine and if you have adequate voltage by connecting your volt meter to your coil + terminal with your switch on, you are not losing ground connection (don't worry about the coil's negative terminal). If you don't have spark; check your points, condenser, rotor and distributor cap.
