Had Germany concentrated more resources and troops in cutting of Malta from supplies/conquering it - how probable was their success?
Germany managed to conquer Crete and in OTL it nearly managed to starve Malta into submission.(Pedestal)
So was it probable or not?
Depending on when it was launched, it would have been virtually certain that the Axis, the Italians would have been heavily involved especially their Navy and Marines, would have conquered Malta. At some points in 1940-1941 the island was so heavily suppressed that it couldn't respond and just waited for the next aerial bombardment.
The question was will. What would the cost be? Probably high. Would the cost be worth the benefits? In the long run based on how the war played out OTL, yes. But the Axis didn't think it would be necessary to take it, because Rommel, as they thought, was just about the take Egypt and Malta would have just starved with a fallen Egypt, making invasion unnecessary.
Of course that's not what happened, so the POD would have been that the Axis leadership halts Rommel from invading Egypt because of supplies and refused to allow Rommel to appropriate the interdicting air assets bombing Malta. From then on, its obvious that the Axis needs to plan for a long campaign, because the Axis is not going to win by conquering. So they will need to take Malta to prevent it from becoming a base against their supply lines.
The Axis would have a better time of it from 1942-1943, when Malta was actually used to really hurt the Axis, but the end is going to be the same, just perhaps the campaign lasting longer and more Axis forces escaping, but this is not guaranteed.