Very low voltage on 18650s
Seems like the cells have protection circuit built in them. When the cell voltages went too low (probably below 2.75V), the protection circuit disconnected the battery. When you connected the battery to power supply, the protection circuit reconnected and you could measure the voltage across it.


  
  Are both these batteries irrevocably damaged and hence should I just dispose of them?
  


First put the cells in the chargers and let them fully charge. If your charger let you adjust charging current, set it to .5Amps. While they are being charged, check if any of the cells are getting warm. If it does the cell is probably damaged.


  2.If so, were these likely to have been faulty cells or did I damage them by leaving them in the gimbal for prolonged periods (or something else)?


Never let 18650s fully discharge by leaving them somewhere connected. Fully draining the batteries is a guaranteed way of damaging them.


  
  Was it unsafe of me to “jump start” the dead battery using my benchtop power supply?
  


Batteries that have entered sleep mode, ie the protection circuit have disconnected due to overdischarge, are often jump started this way. This is known as boosting. But you need to limit the current, or else tha battery may catch fire or explode.