1.) Minimum Crew = absolute minimum for the ship to function. Severe CR penalties and whatnot but technically it can get from A to B. Currently not displayed in the game, beyond seeing the "not enough crew" in the fleet screen. Anything below this number results in the ship being at 0% CR if deployed.
I think the main reason that number is not displayed, or needed, is because you can't manually set how many crew members can stay on one ship. Since the game spreads your crew across the ships, currently the only way to have "minimum" crew on one ship, is to have minimum crew in all ships.I doubt that number could currently serve in anything. The player knows that he/she needs to rush to get crew when they see they are undercrew and normally just mothball some ships, to keep the others in full functionality.
One way I see that number being useful would be, if mothballed ships needed crew. If the player had just enough crew to save some mothballed ships, there should be calculation of which ships would survive with the remaining crew that could be spared. But that requires mothballing to need crew or you lose the ship.
Another way the number would be useful would be if there was a state like mothballing, where you would put the ship in a stabdby state, it would be useless for combat, but as long as it had the minimum crew in that state, it would not lose CR.
The reason I say this is because if you know the number of crew that the ship needs to have 1%CR, so before it reaches 0% CR, do not see why it would be useful. You will never use in battle a ship with CR being in the red, at least not if you have another option, or you like to see what happens when it malfunctions all the time XD