Mealworms can be used as a staple diet...but be sure to feed the mealworms a nutritious diet beforehand so they have plenty of nutrition. You may have to be prepared to buy crickets or other insects once in a while, as leos sometimes will stage "hunger strikes" if fed the same food to often.
I've heard that silkworms are one of the best insect foods you can feed a lizard. They eat mulberry leaves which are high in calcium, and are very healthy...only problem I could see with them is they can be hard to keep and aqquire, unless you have a mulberry tree in your backyard, and they can get huge, bigger than even a leo can eat.
As for calcium, there is something nice about leos....they are pretty "smart" lizards and will moderate their own calcium levels by themselves. Just leave a small bowl of calcium with d3 in their cage at all times...your gecko will lick at it when it feels it needs it. AS for your final question, the mealworm bowl, you want a bowl with clear slides so the gecko can see the mealworms, and be tall enough and slick enough so the worms can't climb out...but not so tall the leo can't get in. For a long time I used a clear box shaped plastic cover I got off of a action figure...cheap, clear, and effective. ^^ Be sure to leave a small layer of food for the mealworms in the bottom of the dish in case your leo doesn't eat them right away. :3