That what I was thinking use subs and the fast minelayers like this one:
http://en.wikipedia.org/wiki/Abdiel_class_minelayer
(which looks like a handy ship BTW)
The Brits could do food only runs to just keep the population afloat, even if you stopped shooting AA and flying Spitfires. However would the Germns just mine all the port entrances and shift to area bombing Valetta to make the civilian population miserable enough to surrender?
Did the civilians have guns? It is the people with guns that make the call, and this likely means a call by the Sea Lords or more likely Churchill. I would lean towards not surrendering unless the UK is looking at leaving the war entirely. I think at some point, the soldiers will stop firing and keep the ammo in reserve for the final battle. Once the ammo is low, the Germans would bomb relentlessly. IMO, if there was no effective counter fire for a few weeks/months, the Germans/Italians likely do some type of amphibious airborne operation
Looking at maps, it looks like Malta has lots of good inlets for submarines and smaller surface craft. I would assume there is enough bad weather, you can sneak a few faster craft in. Say you have 40 hours of darkness or bad weather, this means if timed just right, you could be about 300-400 miles away when you started the run. I bet the craft you listed can bring in 100's of tons of supplies per trip and evacuate 2K to 5K per trip. The evacuation is as critical as the food into the Island.
I may be a bit biased using specially designed merchant submarines, but to play with some numbers. It looks like a submarine can make two trips from Egypt to Malta each month. If you use 15 combat submarines, this is one per day. You have to figure density of the food, but if you only carried torpedoes in the tubes and used all the extra space for food you can get say 50 tons of food per trip and evacuate 100 people. It is not a lot, but 3K off island per month helps a lot. And 50 tons of food per day will feed the soldiers, might even give 700-1000 calories per day to the civilians.
I am less familiar with UK cargo planes, but you likely can get some into the area, if you chose. Or you can just load up a freighter and try to ground it on the beach if desperate. It is an ugly scenario for the people of Malta, but taking the POD of the Germans are willing to commit air power but not any soldiers to taking Malta early in the war, I see starvation deaths, but no surrender. It could very well end up like Leningrad, where say 1/3 of the population starves. But would the UK really give up this base to save on civilians deaths. Maybe, but I would be not. So lets take worse case where Germans start the air operations in late 1940 and it runs full effect for 2 years. UK gets 50 tons of food per day to Island by whatever means (and yes this counts the on had reserves and any local production) and the UK gets of 3K per month. Well, we know the evacuation effort will have movies made about it, but the 72K will not have a huge impact at first. Seems like about 3K calories per pound or 6 million per ton base on stuff I read on WW1 food movements. So we are talking 300 million calories per day for population near 300K. This is about a thousand per day. Once the people run through there body fat in a few months, death occur rapidly over then next few months until calories goes above 1500 per person. So about 1/3 will be starved or evacuated within 6 months or so. We know 18K evacuated, so 82K starve.
But this is near worst case, and the Island stills stand, until attacked. If you can sneak just a few freighters through at say 3K tons per ship, you dramatically lower the death tolls. The UK has a sporting chance of doing this even with more air power. Being a critical base, it likely had substantial food reserves. It is heavily populated, but there is still open land to use. Fisherman likely either shore fish or fish in small craft at night or bad flying weather days. Can't really estimate yield, but every fish helps. So my guess in a more realistic scenario, we see 10K-20K excess deaths. The sick, the old, the young. Births will drop to near zero due to low body fat. With many weak people, we will see some epidemics, so many of the say 15K will not actually starve, but will catch thing like the cold or pneumonia which their weakened body can not fight off. And it is not quite Leningrad, it is not so cold. Being in very cold climates means you need a lot more calories to survive especially if fuel sources are bad.
Now weakened soldiers on half rations will not be able to perform as well, and will have very low morale in most cases, so this is why I see the Germans being very tempted to try to force the issue with an infantry attack.