To prepare stock, remove meat from cooked lobster tail and claws. Chop meat; chill. Place lobster shell in a large zip-top plastic bag. Coarsely crush shell using a meat mallet or rolling pin. Place crushed shell in a large saucepan; add 3 cups water, 2 cups onion, celery, and clam juice. Bring to a simmer over medium heat. Cover and cook 20 minutes. Strain shell mixture through a sieve over a bowl, reserving stock (about 3 cups); discard solids. Place stock in a large saucepan; keep warm over low heat.
To prepare risotto, melt butter in a medium sauté pan over medium heat. Add 3/4 cup onion and garlic; cook 3 minutes or until tender, stirring frequently. Add rice; cook 2 minutes, stirring constantly. Add wine and salt; cook 2 minutes or until liquid is nearly absorbed, stirring constantly. Add stock, 1/2 cup at a time, stirring frequently until each portion of stock is absorbed before adding the next (about 25 minutes total). Stir in reserved lobster meat and corn; cook 2 minutes or until heated. Remove from heat; stir in cheese, green onions, and lemon juice.