OK, now I think I am figuring this out. This unit is not like you have described so my design is different. I understand the seal on the shaft, etc. I made it to the parts supply warehouse today and they show the same parts diagram you do. (above) It does not show the part that is bad on my unit. As a matter of fact it doesn't even show it. All they said is you have to spend 170 bucks for the whole thing, motor and all and that part will be in there even though its not showing.
Lets see if I can explain my disassembly to you. Simple, take out 4 screws around the motor shroud. Take it off. Pull motor out of plastic sleeve and it falls right out in your hand. Now you are looking at the end of the plastic sleeve which supports the seal, macerator and impeller. Take screw out of end of impeller (inside the dishwasher), squeeze clip ears together to release and the plastic sleeve will fall into your hand with the rubber seals around it. From that point there is no need to take it any further apart.
That plastic sleeve is pencil shaped, 2 1/2 inches long is cracked and will not allow the splines on the motor shaft to mate into the plastic splines inside the sleeve and therefore be able to turn the impeller. No impeller turning, no washing, just a motor spinning freely inside a stripped and cracked plastic sleeve. I really hope this rings a bell with somebody and connect me with a way to get this simple part. Once its in my hands I can have this thing together in 10 minutes or less, I guarantee. I really appreciate any info.