Wrong Multimeter connection / floating ground
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. A simplified representation of a digital multimeter schematic. In almost all modern multimeters the voltage range will be auto-ranging and protect itself against over-voltage. (This one doesn't.).

If you plug the "black" lead into the 10 A socket (instead of the COM) while measuring voltage with the red in the red V&Omega; socket you will introduce a small series resistance which will very slightly affect the voltage measurement.

Form Figure 1 we can see that an extra 10 m&Omega; to 100 &Omega; would be introduced between the 10A or mA socket to ground where the V reference is. Compared with the 1 to 10 M&Omega; input impedance of a digital meter the error would be insignificant for most purposes.

The answer to Q1 you can check for yourself without risk of damage.