(1) Emeril Lagasse's recipe (2) myrecipes.com recipe (3) mycookbook.com recipe (4) Southern Living's recipe (5) Straight from the back of the box of Mueller's 
Ingredients Ingredients Ingredients Ingredients Ingredients
6 tablespoons unsalted butter 1   (8-oz.) package elbow macaroni    2 cups macaroni shells 2 cups milk    8 oz. uncooked elbow macaroni
3 cups whole milk 2 tablespoons  butter    3 tablespoons butter 2 tablespoons  butter    1/4 cup margarine or butter
1 teaspoon salt 2 tablespoons  all-purpose flour    3 tablespoons flour 2 tablespoons  all-purpose flour    3 tbsp all purpose flour
1/4 teaspoon ground black pepper 2 cups  milk    2 cups milk 1/2 teaspoon  salt    1/8 tsp dry mustard
Pinch cayenne 1/2 teaspoon  salt    1/2 teaspoon salt 1/4 teaspoon  freshly ground black pepper    1/8 tsp salt (optional)
2 1/2 cups grated Cheddar 1/2 teaspoon  fresh ground black pepper    pepper to taste 1   (10-oz.) block extra sharp Cheddar cheese, shredded    1/8 tsp black pepper
1/2 cup fine bread crumbs 1/4 teaspoon  ground red pepper    2 cups shredded cheddar cheese 1/4 teaspoon  ground red pepper (optional)    2 cups milk
1   (8-oz.) block sharp Cheddar cheese 1 tomato sliced 1/2   (16-oz.) package elbow macaroni, cooked 2 cups (8 oz) shredded sharp Cheddar cheese
1 cup croutons 
         
Measures like 1/2 (16-oz.) package elbow macaroni, or 2 cups macaroni shells are actually equal in weight and volume, so these and other measures and volumes have been converted to a single standard measure.
All-purpose flour will be called flour and margarine/butter/unsalted butter is just called butter.
Milk is milk, which is good old regular whole milk.
An (optional) listing does not appear in the comparisons below.
Ground black pepper and fresh ground pepper are listed as black pepper.
Cayenne and red pepper are the same
Cheddar is cheddar, we aren't looking at its sharpness level, and grated or shredded are the same.
2 cups of cheddar is the same as 8 ounces
teaspoons = tsp.  tablespoons = tbsp. cup(s) = c
Add-in Ingredients means that you can add anything you like to the mac and cheese base recipe---so add lobster or, like the original recipes above, bread crumbs, croutons or tomatoes.
Everything is now converted to a set standard measure, so we can compare accurately---as we compare apples to apples and nuts to nuts!
KEY:
   same in all recipes
  Some similarities in recipe but a different amount of cheese  
  Same ingredient, slight difference in measure
  Not in other recipes
Recipe#1 Recipe#2 Recipe#3 Recipe#4 Recipe#5
8 oz. elbow macaroni    8 oz. elbow macaroni    8 oz. shell macaroni    8 oz. elbow macaroni    8 oz. elbow macaroni   
6 tbsp. butter 2 tablespoons butter    3 tablespoons butter 2 tablespoons  butter    4 tablespoons margarine or butter
6 tbsp. flour 2 tablespoons flour    3 tablespoons flour 2 tablespoons  flour    3 tbsp flour  
3 c milk   2 cups  milk    2 cups milk 2 cups milk    2 cups milk
1 tsp. salt 1/2 teaspoon  salt   1/2 teaspoon salt 1/2 teaspoon  salt    1/8 tsp salt (optional)
1/4 tsp. black pepper 1/2 teaspoon black pepper    black pepper 1/4 teaspoon black pepper    1/8 tsp black pepper
cayenne/red pepper    1/4 teaspoon cayenne-red pepper  1/4 teaspoon  cayenne-red pepper 1/8 tsp dry mustard
10 oz. grated Cheddar 8 oz. grated Cheddar 8 oz. grated Cheddar 10-oz. grated Cheddar 8 oz grated Cheddar
Add-in Ingredients
1/2 cup fine bread crumbs 1 tomato sliced 1 cup croutons 
1 teaspoon Essence

Conclusion:

It seems to me, from my years of experience in the hospitality industry, that recipes #1 and #5 are completely original, perhaps #1 more so because it contains more milk and roux, which will make the entire dish saucier while not making the sauce too thick and it has more cheese too!

Recipe #5 has some dry mustard added but an 1/8 tsp. really won't do much to the flavor. It's like putting a pinch of salt into two cups of water. Also the butter/flour roux is a lot and will make the sauce almost too thick, thus making the dish a bit dry after it's baked.

Looking at recipes #2, 3 and 4, I see the same basic recipe, perhaps tweaked, simply by altering the amount of black pepper in each one. In recipe #3, I see too much butter/flour roux which again will make for a thick sauce.

Recipe #4 is the same as recipe #2 except for the amount of pepper and cheese. More cheese also makes for a thicker sauce while not really adding a lot more flavor. The difference in pepper isn't a big game changer either!

I wonder if Recipe #2 and #4 fits Julia Childs's comments about the tweaking of a recipe---you be the judge as I'm only pointing out facts. I like recipe #1 the best, nice and saucy with good flavor and salt level.