Okay, here's what I have been thinking about. I'm calling it the 3-3-3 plan!
We know you are going to let the Demon Spawn come along. In that case, before you go, you need to have three conversations.
1. With Granny. Tell her you really don't want Blake to come along because when the boys are together, they both become very difficult. Because you realize how important this is to her, you are willing to give this a shot. However, she must agree to let you meter out discipline, and support whatever you decide.
(I'm willing to bet she'll go along with this one. As you said, she didn't let you get away with much when you were a kid. Further, I'm sure she shed no tears when you swatted Blake at KFC.)
2. Speak to you son, and let him know you will be having a conversation with Blake and him together, where you will lay out ground rules for your trip to WDW. Tell your DS that if Blake were not coming, you probably wouldn't be having to lay ground rules. However, you do notice that he picks up a bit of an attitude around Blake, which is totally unacceptable.
(Also, you may want to have a brief talk with Blake's parents, and let them know that if you are taking Blake, you need their agreement that you are able to discipline Blake in the same manner that you do you own son. You will not spank him.
They will be so thrilled to have you take him, they will agree. They know he needs discipline. They are just too lazy or scared to do it.)
3. Right before leaving, sit DS and Blake down together, and lay out your ground rules. These are...
(1) You will treat everyone - adults, CMs, each other - with respect. (You may have to explain to Blake what respect means - no joking.)
(2) You will not have violent or angry outburst or tantrums.
(3) You will do whatever an adult tells you to do, the first time you are told.
To keep track of these rules you will give them each three "tickets". (These can be Mickey stickers, or pins, or Disney Dollars, or whatever. They just have to be cheap and easy to carry.) Whenever a child is found to be breaking a rule, calmly state, "Child, you are punching Granny in the nose (or whatever). That will cost you one ticket. Hand it over, please.".
When a child has relinquished all his tickets for the day, his day is done. Done means done immediatly, not just one more ride. If it's Blake, Granny takes him back. If it's DS, you take him back.
I can promise you that Blake will not make his first day. He'll want to see just how far he can push his luck. If you are firm and calm, and don't give second chances or overlook things , he will catch on very soon. And if he doesn't, hey! You'll still get to enjoy the parks without aggravation.
I have taken this advice straight from John Rosemond's famous 3-ticket plan. I use it occasionally with my own two children, and find it to be very effective.
Please let us know how this shakes out. I've been thinking about you and this kid all day!
Nicole