What is the optimal method for making box macaroni and cheese?
The 4th version you gave is optimal from a chemistry standpoint; the process of using heat to melt a soft fat and dissolve a powder into a liquid by stirring is going to be at it's most efficient when the ingredients can fully interact with each other without all that pasta in the way. 

It seems the main issue at hand here is the 10-15 minute wait for perfection that you mentioned. The thickening agents in the cheese powder need a few minutes to do their molecular bonding magic, and there's nothing we can do to speed that up. All 4 methods you mentioned are going to force this wait for creamy goodness because you aren't starting the sauce until after the macaroni is done.

While I don't have impatient kids to cook for, I do have an occasionally 'hangry' wife so I understand the value of efficiency here. What I would highly reccomend you try (and the method I use at home) for Kraft mac &amp; cheese is basically the 4th one in your question, but instead of waiting until the macaroni is done and using the same pan for the sauce, just grab a second pan and prepare the sauce while the macaroni is cooking. 

Put the second pan over med-low heat and throw in your butter (hard or soft, doesn't matter). After the butter has melted, add the cheese powder and stir until it's mostly wet clumps instead of dry powder. Then add your milk and stir everything together until the powder is dissolved. Just keep an eye on it and occasionally stir while the macaroni finishes cooking. You'll definitely notice it thickening up after a few minutes. When the macaroni is done, drain it and return it to the pot like normal, and stir in your sauce; by this time it should be the perfect consistency or very close. Voila, mac &amp; cheese "aged to perfection" without actually having to wait.