The case length of the British Service .303 cartridge was (in today's units) 55mm long base to end and, of course, rimmed so they look to be as you say. The centre one has a 'separated' case where the case has split around the neck on firing. A common fault on WW2 ammunition manufactured by the millions. Separated cases often resulted in the neck being left in the chamber following extraction resulting in a feed 'jam'.
Yes, a local history website shows the carved tree graffiti left by US serviceman during their R & R trips to the island.
Although a 'range' is indicated as having been located near the island it may have been simply an unobserved area for practice drops under pilot control.