Should I replace this Schottky Diode?
You haven't "shorted" the diode since the voltage is not zero on your multimeter, but you may have cooked it. The "piece of metal" might be a heatsink - without it the diode will overheat.

Your multimeter measurements sound suspicious, but there is a lot of variability in measurement method and multimeters. What you should get is:


Pin 1 - 2: 0.3V
Pin 2 - 1: O.L. within a second
Pin 1 - 3: O.L. within a second
Pin 3 - 1: O.L. within a second
Pin 2 - 3: O.L. within a second
Pin 3 - 2: 0.3V


where pins 1, 2 and 3 could be any of them - you'll have to test all six combinations, work it out from the way it is placed in the circuit, or try to identify the part.

The tests you did with power running are unlikely to give you anything useful. In fact, they're likely to cause damage - the diode mode of a multimeter actually injects voltage so will change the behaviour of the circuit under test. Not only could this activate the diode at the wrong time, it can affect other parts of the circuit that are connected too. If you used voltage mode, then that's different, but you need to be explicit.

The slow rise to O.L. and the low 0.2V the other way is suspicious. It's probably cooked, and if so it may result in the symptoms you describe. It's probably worth replacing it, but given the important role these devices play there may have been collateral damage, so I wouldn't be too confident about that being the fix.