In all likelihood the 9V power supply, wall wart?, is unregulated. A lightly loaded wall wart can easily have a much higher output. Measure the output of the supply to see. The easy solution is to get a regulated supply.
If it were me I would make a little project box with an adjustable automotive grade regulator, and run it straight from the battery. If the combined load from the the meters are less than 100ma, then a 5 pin regulator like the LM2931CT would do, otherwise find a higher rated one. Follow the schematic from the **broken link removed**, using an on/off switch to stop battery drain when not in use.
I cannot speak to the grounding issue, as I have not used those type of meters.