Your mama cat can feed her kittens as offer as she likes. The babies should be fine on a screened porch; if you're too worried about the heat you could set a fan on low for them in the heat of the day so they can get a breeze. And it's perfectly normal for the mama to have some alone time and leave the kittens, because she needs rest to recooperate.
You should hold off on spaying her for at least about 5 weeks or until the kittens are completely weened. Your vet can make a judgment call to make sure she's ready for it.
Your mama cat obviously trusts you with her babies since she'll leave them alone around you, so you shouldn't have to worry about her moving them to another spot - although you should be prepared for it just in case. Some will, some won't - I've had mamas that go both ways. Giving mama some good fatty meat (real meat is better as opposed to canned meat) will help her to stay healthy while she's nursing. You can try good fat like fish and chicken.
Around four weeks you can start introducing the kittens to watered-down Kitten Chow to help them ween. It's very fun to watch them try to figure out how to eat it, but they'll eventually get the idea. And as they get stronger teeth (around 7-8 weeks old) you can start adding less and less water to the Kitten Chow. It is important for the kittens to eat Kitten Chow (as opposed to regular cat food) for at least the first few months of their lives. It has nutrients that will help keep them healthy.
Other than that, enjoy the little guys while you have them - kittens grow up way too fast! Good Luck!!