KMJAB,
Most of your questions are answered at the website www.irp-pri.com and the helpful relocation personnel will explain what you are entitled to during the multiple appointments you will have with them.
But, to make it easy for you:
1. You can use the real estate agent of your choice. The remuneration of said real estate agent is limited to about 5 or 6%, depending on the relocation actual policy at the time. You will have to deal with Royal Lepage Relocation Services to get reimbursed, but you can deal with the real estate agent of your choice.
2. The posting message will state that they move you from somewhere to somewhere else. You can sell the residence that you own at the departing location.
3. I would not recommend selling a year before being posted. Actually, you should not spend a cent of the move until you have a posting message, which establishes the spending authority for the move. If you make any expense without a posting message, it might not be reimbursed. It would be at your own risk. And, last nano-second changes in posting messages are very common. Don't ever believe rumors or career managers: Reality is the posting message, everything else before is just rumors.
Last, take the time to read the documents given to you by RLRS, or the relocation policy at http://www.forces.gc.ca/dgcb/dcba/engraph/CF_Integrated_Relocation_Program0708_e.asp?sidesection=2&sidecat=99
They contain all the details about what you are entitled to during the move.