There is insect screen attached to the fencing. I just used the garage door insect screen that you can get in Home Depot or from Amazon - pretty cheap and very convenient as it has eyelets in it already, making it easy to zip tie to the fencing. Here is the one I used. It lasted for just over 2 years. Will have to replace it now. I could just duct tape the repairs, but it wouldn’t look great, so I am buying new:
Similar to us in southern California I would say it isn’t terribly important, until it is. Sorry for the vague answer, but it just depends. There is some evidence that bees deal with small hive beetles (SHB) better if they get plenty of full sun on the hive.
If you find repeated chalkbrood in a hive, it can help to move it to a sunnier spot.
All of my hives get some shade every day, and my strongest hive probably has only about 3 hours of sun per day. However, they get the sun in the morning, and I think that helps quite a bit with activating them for the day. If you think about colonies before humans, they picked hollow trees. Trees are in forests. Forests are generally pretty shady places. Providing everything else is favorable (good forage, protected from strong winds etc), I don’t see why significant shade wouldn’t be tolerated.