I am currently a Sgt in the Marine Corps reserves. I am strongly considering reenlisting and going active duty. I am currently awaiting orders for voluntary mobilization. Three questions...can I cancel the orders. If I go active duty will that contract override the current orders I have not yet received and finally how likely is it that I can go right away into MSG duty as a B billet with my reenlisting? Can that be part of my contract granted I meet the necessary conditions?

If you go to the recruiters office and let them know you want to augment to active duty they will get the ball rolling.
You will need to get a 368 conditional release from your current unit
You will have to go back through meps but you won't have to take the asvab again.
As long as you pass meps physical it pretty smooth. The augmentation package is submitted and they will let you know if you are approved.
Then you go sign a new contract.
The new contract will supersed the old, and you will not have to worry about the old orders.
If those orders come through prior to you starting the process, then that's another story.
As for Embassy duty, that is something you have to submit a package for and cannot be guaranteed in a contract as far as I've ever seen.
Are you drilling in Illinois, Indiana, or Missouri? out of curiosity