I don't know the answer to your first question.
The normal cycle for a transfer is Request, Send (in transit), Receive (received). I tried it and I was able to cancel a transfer before sending it, but not after sending it -- in that case, I had to complete the process. You can always transfer the items back to their original location with a second transfer.
You cancel a transfer from the "View Transfers" dialog, which is one level up from the dialog that lets you actually create transfers.