Potentiometer not changing voltage reading
In your first test, using two terminals of the pot, the pot and the multimeter form a voltage divider.  The input resistance of a multimeter is typically ~ 10 Megohms, and I expect that the resistance of your potentiometer is much less than that.  With the pot and meter in series across the power supply, they form a votlage divider, and almost all of the supply voltage will be dropped across the meter, due to its very high resistance relative to the potentiometer.